A "New" '67 Solarus combo and a lot of questions.

[aavatech1]

Greetings all,
I recently got my hands on an early Solarus combo, the 7199 / EL34 build w/ 2 x 12" "Harold - Monster Magnet" speakers.
("BG 9/5/67") written on top)

To begin with I've seen that there are several revisions of this amp, many changes and apparently mistakes on some of the schematics.
I've also become aware that some of Sunn's amp designs aren't quite "usual" and that just adds fuel to the bonfire of questions.

I've been working on and learning about audio electronics for a few years now and know what I'm doing inside a tube amp but there always seems
to be situations where there are question I just can't resolve on my own satisfactorily or just wish I had someone to ask about it.
So I really hope and would appreciate those with more experience than me to chime in and lend a hand if possible.

I've got one schematic for the "7199" build and have even checked mine against that and other Solarus versions schematics to sometimes clarify what I'm seeing, or not seeing, so to speak. (i.e. like not being able to tell the correct polarity of any of the electrolytics on any of the schematics)

I replaced all of the old electrolytics along with some of the old carbon comps that had swung way out of spec.
I also replaced the stock transistors which all failed miserably when measured. I ditched the polarity circuit and added a grounded power cable as well. The original photo-cell I kept as it voltage measures correctly.

First question: After replacing the old power cable I connected the ground to an eyelet and that to one bolt of the power trans. The "Red/Yellow" ground of the rectifier secondary to one of the others. Where is the best place, if any, to connect the "Green/Yellow" heater ground? It's currently connected to the chassis riveted terminal strip with the positive side of the 50uF 150V cap, and lead connected to ground of the multi-cap and the "Rate" control ground buss.
The polarity connection of the 50uF cap is also something that keeps bugging me as no polarity is discernible on the schematic. The 50uF bias cap was/is also originally grounded to positive as well.

When I got it it had two 6L6's in it for power tubes and the pre-amp tubes measured quite well except the 7199 which I replaced with a good one.
I replaced the 6L6's with EL34's, per the "7199" schematic.

I then used the voltages from the "7199" schematic to check my voltages and things have come out pretty well, except of course, several points.
I'll attempt to upload the PDF with my current voltage measurements along with the pics of the unit. (hopefully not too many : )
I've read that there are known mistakes on this particular schematic and though I've searched for mention of them I haven't found out what they are yet
So if anyone can help me with known schematic mistakes I can check against my amp's wiring I'd appreciate it greatly.

So right now I've "only" got a few questions/problems. I've uploaded pretty big pics so you can see what I see, pretty much, and in case there are any visible goofs.

This Solarus does currently pass audio. Sine, Square waves. The previous owner had rewired the output jack to make it "default' 16 Ohm and I set it back to the stock configuration of 8 Ohm. They had also installed an RCA jack in the back for some reason but that's history.

So, according to my measurements my first observation is that I'm getting +438 Vdc ("Switch") and +423 Vdc ("A") with the "3=10c" choke in parallel with the first two
multi-cap connections. Voltages which should be around +470 & +460 (spec) respectively.
The "A" connection of the multi-cap, I belive, only feeds the center tap for the Pins 4: (#2 grids) of the EL34's which measure at +417 Vdc and the Pins 3: (plates) +412 Vdc.

The next issues are with the 7199 PI tube. Pins 1,3 & 8 are the only voltages that are off by more than 20%.
All other voltages on pre-amp tubes are either spot on or within 10%
They being: Pin 1: (plate 1) +198 Vdc measured (+220 Vdc spec) / Pin 3: (grid 2) +54 Vdc (+30 Vdc spec) & Pin: 8 (cath1) +103 Vdc (+80 Vdc spec)

All of the resistors in the PI circuit have been replaced with new. (47K 1W x2 - 270K x2 - 330K - 1K x2) both 0.1uF caps have been checked as good.
I'm currently suspecting the electrolytics as they're the only components not new brand new (they measured good and were better than what was there and what I had on hand) The Bias pot is original but the bias adjusts solidly to -34.0 Vdc with less than 0.300 mV variation over time.

The last of my current questions are regarding the pots (...yes, all of them : ) as it looks like the Volume, Bass & Contour pots have been replaced at some point.
The remainder look like the vintage CTS's that I'd expect.
So my question is about which type of Pot is best for which control on this particular Sunn amp. Linear or Log? That is; which for which position.
There seem to be a lot of conflicting ideas out there and with Sunn's idiosyncrasies I thought I'd ask.

I've also 'scoped a 1K Sine and 400 Hz Square wave output from the amp (with 8 Ohm dummy load) to see what I've got going on but maybe it's better to leave that for when the previous issues are ironed out.

This may be a lot but all of this has been offered because I thought it might be helpful to know what I've done and seen so far.
If it's not please disregard.
If there's other or different info that's more important I'd be happy to provide it.

I kind of write like I think so if all of this is meaningless and useless info let me know and I'll do better!
All thoughts and assistance are appreciated. I'm hoping to get better at amp repair and troubleshooting so I too can contribute to the community here.

Regards,

AAVA


PS. Apologies if the pic selection is poor. My first time using this system.

M

[soundmasterg]

Welcome to the forum!

First things first...the Sunn specific section that you clicked on doesn't get as much Sunn traffic as the sub forum does. Here is the link for that: http://sunnforum.ampage.org/

All of the Sunn stuff was either identical to, or based on Dynaco hi-fi amps. The Mk3 and PAM preamp are the usual ones. The earliest Sunn stuff are Dynaco kits in a Sunn box, no joke! The later Sunn stuff once they had a single chassis instead of two chassis were slightly redesigned to be better for musical instrument use, but are still very similar to Dynaco stuff. There are many Sunn amps that are identical except for the name. So you may not find the proper schematic for your amp under one name but if you try another name, you may find that it is closer to what you have. The earlier amps used the EL34 and 7199 setup, and later ones used the 6550 or KT88 and 6AN8 setup. The later amps had higher voltages too. Almost all vintage amps have higher voltages today than when they were built as the AC line voltage is higher now than then. The result is that many of the Sunn amps will have higher B+ than what is shown on the schematic and some versions will exceed the max voltages for the electrolytic caps. Can caps are not as common today as they were when vintage Sunns were made, and are expensive when you find them. Using discrete caps inside the chassis with the Can cap still in place, but electrically disconnected is usually the best approach. It is cheaper for one thing, and the second advantage is that you can increase the voltage rating by connecting caps in series (with parallel resistors) and you can adjust the capacitance higher too. If you go this route make sure to solidly mount the caps to terminal strips. You may have to drill a hole to mount terminal strips. Also be sure if you intend to use a tube rectifier that you do not exceed the max allowable capacitance that those can work into. A GZ34 can support up to 60uF, though modern ones may not do as well there as vintage ones. Never use used electrolytics...its a false economy sort of like getting re-tread tires on your car. Just use new ones and that problem area will be fixed for a decade at least. One of the failure modes of electrolytic caps is that they can leak DC to the chassis, in which case your B+ would be lower than spec.

Never ground anything to power transformer bolts. It is an unsafe practice as vibration and heat can work those bolts loose, or someone down the road might replace the transformer and forget to attach the ground. The safety ground should always be attached to the chassis through its own bolt with a nylock or a keps nut, and left a little longer than the other AC line wires so that if the cord is pulled out the safety ground will be the last wire connected. This is code around the world. You may have to drill a hole in the chassis to add this connection. One of the quirks of vintage Sunns is that it has to be wired a specific way with the AC wiring in order to make the power switch light up. The correct way to wire up the AC section in a tube amp in the US is to have the hot wire go from the wall to a fuse and then a switch before connecting to the transformer. The neutral side of the line gets connected directly to the transformer, and the safety ground is grounded on its own bolt as mentioned previously. You can see a good example here (along with many other great tutorials) http://www.valvewizard.co.uk/standby.html.

Bias supplies are a negative supply, so the capacitors are reversed, with positive on an electrolytic connected to ground. Anything grounded to that cap's positive side should remain that way, however a heater center tap should be connected to chassis ground or to a positive voltage to float the heaters away from ground which reduces hum. The vintage way is to ground the heater center tap to chassis ground. Connecting it to a positive (or negative) voltage is a well known trick to reduce hum, but the positive direction is safer for the tubes ratings. Be sure that wire is in fact the heater center tap before you go moving it around. It might be ok where it is though as if it is connected to the positive side of the bias cap, that point is chassis ground.

I can't check the schematic and your pictures right now, but those voltages might be ok, or slightly low. The 7199 voltages are often all over the place so don't stress those as long as they pass signal well. Likewise on the pots, I don't recall what they used in Sunns, though I do remember they are different than the Fender usual regarding the linear/log thing. I also remember Conrad Sundholm (Sunn co-founder) telling me something about the pot choices (linear vs log) but I don't remember what he said as it was probably 10 years ago that we were talking about it. I'll have to see if I can remember what was said when I have a little time to look over the schematic.

What kind of undistorted power output is your amp putting out? Hopefully you have it connected to a resistive load and not a speaker, and have a True RMS meter to measure the power output? Knowing that will help us to know if the amp is functioning as it should or if it is down on power.

Greg

[soundmasterg]

A couple more things about Sunns....they are all UL as in ultralinear and they all have negative feedback. Great for hi-fi and bass, not as good for guitar. I don't like Sunns personally for guitar but YMMV. To me they are loud, clean and when distorted the distortion is hard without many pleasant overtones. Some of that can be solved by disconnecting the NFB, but even with that I still don't like them for guitar. I like them for bass fine though.

Sunns also use a choke input power supply instead of a capacitor input. So the choke comes first and all the current in the amp runs through it. You mentioned the following:

So, according to my measurements my first observation is that I'm getting +438 Vdc ("Switch") and +423 Vdc ("A") with the "3=10c" choke in parallel with the first two multi-cap connections. Voltages which should be around +470 & +460 (spec) respectively. The "A" connection of the multi-cap, I belive, only feeds the center tap for the Pins 4: (#2 grids) of the EL34's which measure at +417 Vdc and the Pins 3: (plates) +412 Vdc.

Are you sure the choke is in parallel with those caps? Each cap should be going to ground with the choke in between them. See the schematic I attached. Connection point A on the schematic goes to the center tap of the output transformer, and since the amp is UL, this supplies the plates (pin 3) and the screens (pin 4) for the power tubes. Section B supplies the cathodyne inverter (the triode section of the 7199) and the recovery tube for the reverb (1/2 12AU7) and Section C supplies the gain stage preceding the phase inverter (the pentode section of the 7199) and the rest of the preamp and reverb.

Your voltages do seem to be a little low on the power tubes. Replace all electrolytics with NEW ones, wired correctly. Then check the voltages again. The first Sunn I worked on, a Sonic 1 (identical to a 200S) had B+ voltages around 390V and they should have been 560V or so. As the amp was on longer that voltage would slowly drop too. A sure sign of bad electrolytic caps leaking DC to ground. I left the can cap on the chassis for cosmetics but disconnected it electrically and used terminal strips and discrete 350V caps connected in series with 220k resistors in parallel with each series cap to get a voltage rating of 700V and no more problem. There could also possibly be a problem with the choke too, but I would try the cap replacement with new caps first before trying a different choke.

Greg

[Enzo]

The schematic you posted has a cap input filter, not choke input.

[aavatech1]

Hi Greg,
I can't thank you enough for your detailed reply to my post.
My reply to you will be a bit more terse than my previous attempt last night so I don't loose it all again.

I'll also note that in my original post I forgot to mention that I had not only replaced all of the electrolytics but had replaced the original multi-cap with a new CE Mfg. 30/20/20/20uF / 525Vdc multi-cap as well. Apologies.

"Never ground anything to power transformer bolts. It is an unsafe practice.."
I made the changes to the power cord grounding as recommended. A swap of the "Neutral" and "Hot" leads and a switching of the leads to the switch and I've got a correct US config and power light. I have also removed the power cord ground from the AC transformer bolt and will drill a dedicated hole to mount it correctly.
The "Red/Yellow" ground lead from the AC transformer secondary (Rectifier winding) is also soldered to an adjacent eyelet and bolted to the one of the AC trans' bolts that will be moved as well.

"Never use used electrolytics...its a false economy..."
In regard to the negative bias circuit I thank you for the information. The two 50uF 150V electrolytics I have are installed correctly. But since I didn't have new ones on hand I used some 80uF 150 Spragues that I did have. I've since ordered new replacements of the correct values.
The "Green/Yellow" ground wire I mentioned is indeed the heater winding ground is also connected to the correct point.j

"I don't recall what they used in Sunns, though I do remember they are different than the Fender usual regarding the linear/log thing."

In regard to the Pots questions I'll hold off on that for the moment in lieu of ironing out the voltage issues and in hopes of you having a Conrad flashback or someone else chiming in with Sunn specific information.
It must have been great to have met Conrad Sundholm. He probably had a lot of super stories to tell! There really should be more about him online.

What kind of undistorted power output is your amp putting out?

My first signal test was with a 1kHz Sine at 150mV. With the output running into an 8 Ohm dummy load and 'scope leads connected at the load.
I got a clean Sine to about 16 VAC-PP and at that point abrupt Low Hz loss and phase shifting. So, clean to 16VAC-PP (16*16)=256/8=32W into a non reactive load. Is this a correct interpretation?
I've yet to look at Square wave input as of yet but when the Sine starts to distort it starts to look more like a Square wave w/ sharp low Hz loss and phase shifting.
I'm not sure if folks would think that it's a bit too early to be looking at this stuff considering still having voltage anomalies but I was curious.

"..about Sunns....they are all UL as in ultralinear and they all have negative feedback"

I've been reading up on the differences between UL designs and "traditional" push-pull EL-34 designs. I have vague memories of someone saying that one of the mistakes on the "7199" schematic, at least, is the representation of the feedback wiring. I'm somewhat of the mind that "it is what it is" more so than trying to make it sound like what I'd like it to sound like at this point but I am interested in if the feedback circuit shown is incorrect or not and making it so if it's not.
I'd like know what it's designed to sound like before I go changing anything. ; )

"Are you sure the choke is in parallel with those caps? "

I believe so.
One lead of the choke is connected to the 30uF tap of the multi-cap and the other is connected to the "A" 20uF tap of the multi-cap where the 6.8 Ohm is connected to the next 20uF tap ("C"). It is correctly wired to the red lead of the output transformer.

"Your voltages do seem to be a little low on the power tubes"
As I mentioned previously I've ordered new 50uF 150V electrolytics for the bias circuit and will remeasure voltages again when they're installed.
Leakage from the Sprague caps I had on hand is indeed possible. Honestly I have no idea how long they're been around here. They measured well but that means little outside of a circuit, really.
When I installed the new multi-cap I soldered the twist-tabs of the 'cap case to the chassis the way it had been done originally.
Two of the other 3 twist-tabs are connected to other chassis ground points.

Again, Greg, thanks so very much for your input. It's given me a LOT to think about and research. When I get the new bias caps in I'll be back with new voltage measurements.

Thanks also to "ENZO" for your input as well, though, I'm not sure about your description of the "cap input filter".
There seems to be a discrepancy between what you're saying and what "Greg" has stated. Are you referring to the first section of the multi-cap, which is in parallel with the choke?
Can you clarify?

Thanks, again

AAVAX
Mark

[soundmasterg]

The schematic you posted has a cap input filter, not choke input.

Woops, thanks for the correction Enzo! I shouldn't post right before I am going to go to sleep....the brain stops working an hour before!

I should have said that Sunns use a choke that everything including the B+ runs through, which is different than many vintage amps. Sorry to lead anyone astray!

Greg

[soundmasterg]

Hi Mark,

Please ignore what I said originally about the choke input vs cap input on the power supply. I posted right before going to sleep and wasn't paying attention to the schematic close enough. Enzo is correct. The only difference from the norm there is that the Sunn has the large choke that all the voltages going through, including the B+. Many vintage amps that used a choke have the choke between the plate and the screen but no overall B+ goes through it in that location. Read this link for more info on the differences. http://www.aikenamps.com/index.php/chokes-explained

Sounds good on the grounding changes and the electrolytics. It would work the other way you had it but proper grounding is something that you should always be as safe as possible with. The can cap is overpriced as I mentioned previously, but it will likely work ok. Your amp has lower voltages with the EL34 circuit than the 6550/KT88 circuit does, so once you get the voltages worked out correctly it will likely still be under the voltage limit of the cap can. 32 watts doesn't sound like it is putting out what it should though. The bigger two power tube Sunns were 60-70 watts RMS, and the EL34 ones were 40-50 from what I remember. Hopefully the bias caps will correct the voltage issues.

I'd like know what it's designed to sound like before I go changing anything. ; )

Sunn started from Conrad supplying his brother Norm with louder and cleaner bass amps and cabinets. Norm played bass with the Kingsmen (Louie, Louie) and his Fender wasn't cutting it, so Conrad had a dream about the cabinet and made one, then built a Dynaco kit amp to get more power and netter sound. The Dynaco transformer equipped early 2000S that I have amplifies cleanly down to 10Hz, largely because of the huge size and good quality of the output transformer. The later amps used Schumacher transformers and they weren't quite as nice as the Dynaco ones, but the amps had slightly higher voltages so they put out a little more power than the Dynaco amps. I'm talking 1968 - 1969 as the transition. By 1970 all the Sunn tube amps were using Schumacher transformers. Anyway, the basics are that Sunn was designed to be clean and loud, with a wide frequency response, and since they are based on hifi amps, they are very clear. You can really hear all the notes that are being played in detail as opposed to the notes blurring together. That is especially the case when using the original JBL speakers that they came with. For me personally I like that on bass but not for guitar.

One lead of the choke is connected to the 30uF tap of the multi-cap and the other is connected to the "A" 20uF tap of the multi-cap where the 6.8 Ohm is connected to the next 20uF tap ("C"). It is correctly wired to the red lead of the output transformer.

If the choke was in parallel with the caps, then one lead of the choke would connect to one of the positive connections on the cap and the other end would go to ground.....in which case it wouldn't work very well. So the choke is in series with the B+ so all the B+ passes through it but it is not in parallel with the caps. So it is wired correctly.

It must have been great to have met Conrad Sundholm. He probably had a lot of super stories to tell! There really should be more about him online.

Yeah Conrad is a great guy and he does have some great stories, including a really funny one about when he met Keith Moon! I think Conrad is fine with laying low and not having a bunch of stuff online about him. He sold Sunn around 1973-1974 and moved on to other things for a long time before he came back to building Conrad tube amps. Now he is mostly retired and is enjoying life.

Greg

[aavatech1]

Hey Greg,
thanks again for the response.

"Please ignore what I said originally...... Enzo is correct."

Thanks for the link for info on chokes and thanks to "Enzo" for the good catch on the choke config.

"..32 watts doesn't sound like it is putting out what it should though.'

The product of 32 watts was just the output before the Sine wave started to distort drastically. It was not at 100% volume, ..I believe.
At the time I was paying more attention to the possible premature "corruption' of the signal than total output.
Kinda just checking to see what was going on. But once I get the new caps in and get things ironed out
I'll 'scope it again and maybe we can get into different aspects of the results. In the meantime, more reading!

"If the choke was in parallel with the caps..."

Yup, fubar on my part.


"Yeah Conrad is a great guy..."

Boy, I'd like to hear that Keith Moon story sometime! Someone should get Conrad to write a book one day.
I bet it'd be a hum-dinger!

Thanks again Greg,
...more when I get the discussed changes made.


-Mark>

[soundmasterg]

The product of 32 watts was just the output before the Sine wave started to distort drastically. It was not at 100% volume, ..I believe.
At the time I was paying more attention to the possible premature "corruption' of the signal than total output.
Kinda just checking to see what was going on. But once I get the new caps in and get things ironed out
I'll 'scope it again and maybe we can get into different aspects of the results. In the meantime, more reading!

Hi Mark,

the RMS power should be measured with a little distortion, if I remember correctly it is 5%. I don't have a distortion meter so in the past when I have measured the RMS voltage on the output with a trueRMS meter and the scope showing a clean sine wave just before it starts to distort, the numbers that I get are a little lower than what the amp should put out. I suspect I am measuring at too low of a distortion level, but it is enough to give me a good idea of what is going on and if the amp is working as it should. The true RMS numbers are not the same as the amp's max power level at full tilt. A Vox AC30 is about 33 watts RMS but is like 60 watts at full tilt. Also, the power output should be measured when the amp is going into a resistive load rather than a speaker, and the power level out will change with frequency. 1kHz is a common one to measure power level at...another is probably 400Hz and maybe 20kHz, but guitar amps often won't go that high though many hi-fi amps will.

The Conrad Keith Moon story isn't appropriate for public consumption. Actually its not too bad but I wouldn't want to mangle the story. Conrad tells it well though and it is really funny.

Greg

[loudthud]

The 32 Watt measurement could just be that you were connected to the wrong impedance tap. Go to this thread:

http://sunnforum.ampage.org/index.ph....html#msg18880

You will have to join the forum to see the pictures posted there. In the mean time, most Sunn amps conform to this color scheme:

Impedance.......40W/60W.......120W

16 ohms...........yellow..........yellow

8 ohms.............orange.........white

4 ohms.............brown..........brown

Ground.............black...........black

[aavatech1]

Hi "loudthud",
Thanks for the info.
The impedance wiring and the "FB" circuit is a subject that after a LOT of thought I had convinced myself that I had settled. At least for the time being. And of course this really wasn't and still isn't the case.
In reality because of having heard that there was something wrong about the "FB" circuit's layout on the Solarus schematic and the fact that it is indeed differently wired in my amp I remain somewhat..perplexed. Even if now only slightly so.... because it works, it still makes me uneasy every time I look at that schematic.
I've included pics of the way I have it hooked up currently. This was achieved when I first got this with the assistance of the " '69 Scepter" photo you linked to and the "7199' Solarus schematic.
In short: someone had added another jack and had it wired "funny". So I attempted to put it back the way it's "supposed to be".
That being like the "Orange: 8 Ohm" & "Yellow: 16 Ohm" config shown in the " '69 Scepter" photo. The two amps are similar in their color coding at least. But the schematics are indeed different at the output jacks in the schematics I've got.

On the back of my amp are "Speaker" and "External Speaker" jacks. So I have it wired "like the Scepter" since my cabinets impedance is about 12.6 ohms nominal. ( 2 X 8 ohm/series/=16 ohm cfg.) That is why, I believe, it's correctly connected in the 16 ohm winding for when the cabinet it plugged into the "Speaker" jack. By the way, this speaker jack is a real jem. At first I thought you could run the "combo" speakers with another "External" cabinet. No such luck.
It seems to be one or the other. Plug into the "Speaker" jack; you get 16 ohm tap. Plug another cab into the "External Speaker" jack it "lifts" the 16 ohm winding and gives you the 8 ohm tap. So Conrad says "take your pick" ...but you can't have your cake and eat it too!
(..I think the guy that had this before me was probably a big cake fan given that 3rd 1/4" in. jack that I took out) Anyway...

Back to the issue at hand; (..segue between the output wiring and the "FB" circuit wiring)
Not being sure about the "FB" diagram on the Solarus "7199" schematic I left the majority of the existing wiring in place since it seemed to make more sense than what I was seeing on the schematic but then also used to schematics jack circuitry wiring along with the pic of the Scepter with the similar (color sceme) wiring.
What concerns me is the correct way to wire the "FB" circuit. I may have it wrong so I've included some pictures of that as well.
As a description of them, and this may not be all correct so forgive me ahead of time, what I'm seeing is:
The EL-34 tube that the "FB" wiring is added to has the "390pF" cap series connected between the #4 pin (G2) and the not used #6 pin.
There is a yellow wire that then heads to the intersection of the 470 Ohm & 680 Ohm resistors. Another yellow wire then leaves that point and heads to the terminal strip where in meets a 1K 1/2 W resistor which is wired in series with another yellow wire (of course) which heads to the yellow 16 Ohm winding's connection to the jack. Wired, I belive, so that when a plug is inserted into the "Speaker" jack the 8 ohm tab is lifted and the 16 ohm winding's tab makes contact. (..oddly enough wired just like the Scepter's jack's wiring without the 750pF cap in parallel with the resistor.)

O.K. so, it seems to me that the schematic is "wrong" but, not being an expert on Sunn's implementation of UL circuit design, what exactly is supposed to be "right"?! I have to admit that I can't get my head to 100% sure about this. I have looked through the forum but still haven't come up with an answer.
Please forgive me if it's there somewhere and I missed it.

So please let me know what you think if you will.

Thanks again to any and all who would chime in on this. I'm learning a lot!

AAVAX
Mark

[loudthud]

Your post is hard to read because you insert too many "carriage returns" (sometimes called the Enter key). At the end of a sentence, simply insert a space and continue with the next sentence. At the end of a paragraph, insert two carriage returns to start a new paragraph and create a blank line between for clarity. This makes your post easy to read on any computer or device regardless of how big the display is.

What you call a 470 Ohm resistor is 47 Ohms. Yellow-Violet-Black-(tolerance band). I think your feedback wiring is correct. The only issue is the 330K resistor shown on the schematic posted above. I think that resistor is the error in the schematic and should be left out.

Your speakers seem to be wired out of phase with the red dots connected together, but it's hard to tell because the photo is too dark making the black wire hard to see.

There is still the question of why you only measured 30 Watts. Describe your procedure and equipment you used to make the measurement. I wouldn't expect 40 Watts out of some old 6L6GC's in this circuit.

[J M Fahey]

First of all, welcome to the Forum

Your speakers seem to be wired out of phase with the red dots connected together, but it's hard to tell because the photo is too dark making the black wire hard to see.

Good catch.
Yes, red dots are joined by the right side white wire, black wire from jack goes to bottom "-" terminal, white wire from jack goes to top "-" terminal.

Black wire is patched at the correct length to make me think that cabinet was originally 4 ohm, both speakers in parallel, red to red joined by a white wire, "- to -" joined by a black wire, and were improperly converted to 16 ohm by leaving reds joined, jack/long black wire to bottom one ("hey!!! it came that way from Factory !!!"), "- to -" short black wire was disconnected from bottom speaker and joined to black coming from jack.
It beats me how could anybody play more than 30 seconds into that cabinet and not notice something is very wrong.
Sort of bearable with a Guitar, but ... on BASS?

There is still the question of why you only measured 30 Watts. Describe your procedure and equipment you used to make the measurement. I wouldn't expect 40 Watts out of some old 6L6GC's in this circuit.

I see a weird mix of measurement units:

I got a clean Sine to about 16 VAC-PP and at that point abrupt Low Hz loss and phase shifting. So, clean to 16VAC-PP (16*16)=256/8=32W into a non reactive load. Is this a correct interpretation?

To measure power output and use the Math above, the volts used must be RMS, and are normally measured with a multimeter (Digital or Analog) .

If you use a scope instead, you will see Vpp shown on the screen.
You can use them for measurement, of course, but they must be converted to RMS, dividing them by 2.83

You seem to be showing both (different) units mixed at the same time, so please confirm whether they are "VAC" which normally mean RMS or VPP which indicate Peak to Peak.

As of the somewhat reduced voltage and power, maaaaayyyybbbeeee you are using new tubes (EL34 in this case) but old/weak GZ34 .

As Enzo often says: "the amplifier is just that thingie between the power supply and the speaker" which is true: the amp can not deliver what was not fed to it in the first place.
Weak supply: weak amp.

As mentioned above, old amps designed for 115V or so typically show higher voltages today because of higher mains .... but yours shows significantly lower, so ... there must be some reason for that.

[aavatech1]

Note: This was a reply to "loudthud"'s post and has apparently shown up out of order. Apologies.

Hey Loudthud,
sorry for the excessive "carrage returns". You're indeed correct about the 47 ohm resistor. Trying to write at 1:00 AM has its hazards. As I mentioned above I'm not an engineer so I'm curious as to why you believe the 330K resistor is a mistake on the schematic. It was there when I received the amp and I just replaced the drifted carbon comp that was there. I'm getting the correct voltage (+1.1 Vdc) on pin 6 (K2) but I've got +55 Vdc on pin 3 (G2) spec'd at +30 Vdc. And pin 8 (K1) is at +105 Vdc when it's spec'd at +80 Vdc. +20 Vdc over spec'd voltage on both; coincidence? "soundmasterg" says that "7199" tubes can be "all over the place" but I can't help but wonder.

You (and Mr. Fahey above) are correct about the speakers being wired out of phase. I hadn't caught that because I really havn't dealt with the cabinet yet. I've only opened it up for the second time to take the pic of the inside but didn't notice the first time or last night. Thanks much, good call. I removed the 6L6's that were in here when I got it and installed some NOS Sylania fat bottle EL-34's. Please see my reply to Mr. Fahey (above) for my method of measurement and the mistake I made in calculating the output power.
Thanks agian Loudthud, I appreciate your help.

Mark

[aavatech1]

"Yes, red dots are joined by the right side white wire, black wire from jack goes to bottom "-" terminal, white wire from jack goes to top "-" terminal."

Mr. Fahey, thank you also for the info on correct cab wiring. As I mentioned above I'd only measured the impedance of the cab for reference but hadn't inspected it closely enough. Lesson learned. Will remedy immediately. You are indeed right my nomenclature, methodology and math were incorrect as I only measured the Sine output on a scope and did not convert to RMS. ...corrected: a scope measured 16 VPP (16*16=256/2.83=90.45/8=11.3 W). Again that was just a measurement I took (albeit incorrectly) to see at what power the amp started to show the drastic distortion I noticed, noted and mentioned above. As I re-read my first post I realized that I had made a mistake. I had originally stated the I had re-wired the output ("Speaker") jack for 8 Ohms when in reality I stated it backward. It was wired for 8 ohms (..with that weird 3rd jack previously installed) and I re-wired it per the schematic to the 16 ohm winding ("Speaker" jack).

Thank you also for pointing out the difference between AC measurements taken with a DVM and an oscilloscope. I found that kind of funny because my DVM IS a scope-meter (Fluke 124) and the AC voltages are converted automatically to RMS values. Go figure. But to clarify the measurements taken and commented on originally where indeed taken on a traditional oscilloscope.
Again, my gratitude for the education. Generally I try to take issues one at a time but in regard to power measurements of tube guitar amps; my understanding is that they are "designed" to distort at some point. So at which point (amount of output power) are they generally supposed to start distorting? ( either a Sine or Square wave.) I understand that the two different waves are useful for measuring different aspects of an amps functioning and or functionality but obviously need to study further on the topic.


"..you are using new tubes (EL34 in this case) but old/weak GZ34 ."
You are correct sir in that the GZ34 is what was in here originally. I replaced the installed RCA 5AR4/GZ34 with an NOS International Service manster 5AR4/GZ34 (labeled "Japan")..but the voltages to the multi-cap are the same. Currently the EL-34 plate and G2 voltages are up equally by ~ 4 Vdc. All else remains the same. I'm aware of the differences between line voltage values past and present and from place to place but when I'm working on a vintage tube amp I use a Variac with an amp and voltage meter that has the voltage set to 117 VAC until things get ironed out. It looks like it's still just the first two multi-cap points ("Switch" & "A") that are low in supply. The points which have the choke connected between them. On the GZ-34 I've got 374 VAC on the plates (pins 4 & 6) and 432 Vdc on (H,K) pins 2 & 8 . I increased to the voltage to 120 VAC and measured 450 on the plates but started getting an odd reading from the H,K of the rectifier. (...I think I'll go pick up a new 5AR4. Old tubes are great but...) I'll continue when I pick up the new rectifier.

Thanks again Mr. Fahey for your contribution, it's greatly appreciated.

-Mark>