parameters for push pull class A output?

[cgiff]

Cathode bias output

Gents,
I built a pair of Matchless Chieftains reverbs out of Traynor Bassmasters. #1 is a 68Traynor, Hammond xfmr supposedly equivalent to 1650N (z~4.3K) with a pair of el34's, plate ~400V ~390 cathode resistor, 1K screen grid R through a toggle for pentode triode operation, 5aR4 rectifier. It's been 4 years since I've opened up the chassis. So some of the voltage details are cloudy I did a post a couple of years back that went into a lengthy detail of bias values sound etc. To my ears and many others this amp, pre and power is the shit!
Changing speaker impedences and tubes does not have near as much effect as changing speakers and cabinet styles ie JBL, Celestions, Jensen (all 8 Ohms) open, closed back cabs and yes, Guitars and pickups.!.

#2 was built from a 1964 -65 Traynor. Much larger power transformer than the one above. It has a 5 volt winding but no room in the chassis for a rectifier. This guy has 4 el34's each with a ~390 cathode resistor, 1K screen grids through a 4PDT toggle for pentode triode operation. The plate voltage from this xfmr is higher through the diodes, so I ended up using a power resistor to get the plate voltage into the 400 volt range with diode rectifiers. The original output transformer was removed. I used a Hammond1650R (Z ~5K) on this amp. It has noticebly more power than the dual el34 amp above. Similar, but not as sweet sounding as #1.
The difference in sound of super hot bias vs a warm bias IMHO is a lot of bunk. I experimented like crazy using 4PDT toggle switches for a/b bias comparisons. Picked a bias that was within safe operating limits of the OP's and have'nt opened up the boxes since. I play a Lado with Dimarzio's a 335 with PAF's and a Jap Strat. through Belden coax cables. Straight into the amps, no shit in the way of the signal.
Aslo did a Gibson GA30 cathode biased 6V6 output, but with Matchless DC30 preamps 12ax7 and El34 preamps.
same observation as above speakers make a much larger impact on sound than bias and tubes.
Point I'm trying to make is don't kill yourself trying to find subtle sound changes in iron when the input and output transducers play a much larger role in the sound process.
Just my 2 cents
Cgiff

[Alex/Tubewonder]

Matt,
I think I misunderstood your post. Yes, OT from an AC30 can be used with EL34 tubes. As to where the Raa values come from: standard design practise is to aim for maximum power transfer i.e. maximal output power while still staying in chosen class of operation. There is no law saying that it's mandatory to aim for max power output.
The traditional way of finding proper values for Raa and tubes' static operating point is to draw load lines on the plate characteristic graphs. The fun begins when you realize that your real life load, the loudspeaker, has an impedance that varies with frequency and your straight load line becomes an ellipse. But for the sake of argument and simplicity we assume a nominal speaker impedance when estimating optimum Raa.
There are some classic articles by Crowhurst explaining all this in great detail.
They are HERE - scroll half way down to "Designing Your Own Amplifier". Recommended reading.

[Alex/Tubewonder]

+1

[Mouthurst]

Gents,
I built a pair of Matchless Chieftains reverbs out of Traynor Bassmasters. #1 is a 68Traynor, Hammond xfmr supposedly equivalent to 1650N (z~4.3K) with a pair of el34's, plate ~400V ~390 cathode resistor, 1K screen grid R through a toggle for pentode triode operation, 5aR4 rectifier. It's been 4 years since I've opened up the chassis. So some of the voltage details are cloudy I did a post a couple of years back that went into a lengthy detail of bias values sound etc. To my ears and many others this amp, pre and power is the shit!
Changing speaker impedences and tubes does not have near as much effect as changing speakers and cabinet styles ie JBL, Celestions, Jensen (all 8 Ohms) open, closed back cabs and yes, Guitars and pickups.!.

#2 was built from a 1964 -65 Traynor. Much larger power transformer than the one above. It has a 5 volt winding but no room in the chassis for a rectifier. This guy has 4 el34's each with a ~390 cathode resistor, 1K screen grids through a 4PDT toggle for pentode triode operation. The plate voltage from this xfmr is higher through the diodes, so I ended up using a power resistor to get the plate voltage into the 400 volt range with diode rectifiers. The original output transformer was removed. I used a Hammond1650R (Z ~5K) on this amp. It has noticebly more power than the dual el34 amp above. Similar, but not as sweet sounding as #1.
The difference in sound of super hot bias vs a warm bias IMHO is a lot of bunk. I experimented like crazy using 4PDT toggle switches for a/b bias comparisons. Picked a bias that was within safe operating limits of the OP's and have'nt opened up the boxes since. I play a Lado with Dimarzio's a 335 with PAF's and a Jap Strat. through Belden coax cables. Straight into the amps, no shit in the way of the signal.
Aslo did a Gibson GA30 cathode biased 6V6 output, but with Matchless DC30 preamps 12ax7 and El34 preamps.
same observation as above speakers make a much larger impact on sound than bias and tubes.
Point I'm trying to make is don't kill yourself trying to find subtle sound changes in iron when the input and output transducers play a much larger role in the sound process.
Just my 2 cents
Cgiff

Hi Cgiff- I thought Id ask since you have experience with the Matchless stuff.
Now this may sound stupid but...I was wondering, I have a matchless clubman circuit with 2x el34's and a higher than normal B+-[B]532, also a larger OPT-each tube has separate cathode biasing resistor scircuit and it apparently was making roughly 48 watts and running too hot and sounds incredible.
Now the problem is it eats OP tubes like no tomorrow. They are slowly getting worse in the red glow department and quicly getting pricey in the wallet.
I added a new EH el34 but now the other old one is going really red and yikes!... the new on is starting to glow red in parts too.

...the result of it running to hot ? If so; will adding more ohms to the cathode resistors lower the current to the plates resulting in less tubal anhilation? Or does it work the other way round?

added...In this type configuration does having matched tubes help?

Anyone else feel free to chime in, hope Im not hijacking this thread...but I think

Now I haven't much gear save a soldering iron and I can get some resistors-
The thing was biased nearly class A when it was finished, the flellow who helped me wire it is no longer avaialble but he said I could run 6550's in it which would bring it up to 55watts.
Any ideas?
The El34's are great but I liked the sound of the American style tubes in it, I used 6L6GC's but was worried it would eat them too.
Would putting 6550's in it, would that soak up some of the heat or should I add some more Ohms in the Cathode resistor circuits.
If using the heavier tube is an option should they be matched?
I suppose it could be some other problem but I thought Id look there first.
thanks -
William (tube electronics novice)

[Steve Conner]

I'd say you definitely need to increase your cathode bias resistors until the tubes stop red plating. Or lower the B+ using some kind of power resistor or transistor/zener thing.

[Ray Ivers]

William,

What's your budget? A big improvement in tube lifespan/red-plating reduction (along with both greater power-amp overdrive capabilities and increased clean headroom) can be had for about $30, and an additional $15 will get you even cooler/longer tube operation. Steve's suggestions are also quite good IMO.

Ray

[Mouthurst]

Thanks Steve and Ray- Ill increase the resistors untill they stop red plating. Im may consult another person I know whom is quite knowledgable.

It was working great then I began using a super efficient celestion century 80 neodynium speaker and I guess it didnt have to work as hard or something. Something changed anyways.
thanks.


heres a pic cool eh? I built it and then had it wired by a local expert.

[Mouthurst]

William,

What's your budget? A big improvement in tube lifespan/red-plating reduction (along with both greater power-amp overdrive capabilities and increased clean headroom) can be had for about $30, and an additional $15 will get you even cooler/longer tube operation. Steve's suggestions are also quite good IMO.

Ray

Well I do have a bit to spend on this amp as it has a great sound. I'd be willing to put that out as long as it didnt alter the tone too much, although I suppose it would still sound good.
With the cost of tubes, If its a mod that'll make it better, why not, I guess its worth the 45 bucks for sure.

What would you propose? Please message me if you're still interested.

I thought about some kind of bias control pot using the 1 ohm thing with test points. I can put the test resistors in but Im not sure how to or what value the pot should be.
I have a couple of 5K pots from the original pwr amp and also have a couple of heavy DC 5 ohm/6A.

Im not sure if I should just balance the resistance so they are identical using matched tubes or??? I was told that using matched tubes would correct the problem but Im not sure I want to lay out the bread if it didnt work.
I have to closely matched 6l6GC's and they dont seem to glow but I didnt use them too long as I dont want to wreck them too.

Im using a 2204 marshall I built myself right now so I have a bit of skills and knowledge. I just dont know which the best option is since the Match 50 is my favorite amp of all time in the whole world ha-ha-ha- I dont want to mess it up and blah blah blah...

So yeah Im interested you could email me if you like and thanks for the help.

[Ray Ivers]

William,

The stock Clubman circuit runs the EL34's at about 148% plate dissipation, and your amp (with a much higher B+) undoubtedly far exceeds this. I'm amazed your tubes survive more than an hour. I hadn't 'done the math' prior to my earlier post, so now I've revised my recommendations a bit.

If your main goal is to keep the exact sound you're getting now, your options are limited; you could try fan cooling (the $15 fan at the link below is very small, and runs on 120VAC):

http://www.alliedelec.com/Search/Sea...5A58806CF5E17F

This may or may not allow you to squeeze a bit more time out of your tubes before they melt down. If you had a small "muffin"-type fan lying around, you could use this for testing.

Since I would never design an amp like this, any further suggestions of mine would run more along the lines of a re-design - which would definitely change the sound of the amp - so I would suggest trying fan cooling and see what effect that has; in conjunction with increasing the cathode resistor values, you might be able to drop those tubes down from unbelievably-flaming-hot to just super-smoking-hot. Bringing your tubes down into what I would consider a 'borderline-acceptable' range (100% P.D. - YMMV) would require quite a bit more work.

Ray

PS: Designing an amp to be easily serviced can tremendously increase the difficulty of construction; I know from personal experience.

[Steve Conner]

Looks like your choke is overheating and leaking wax too. Lowering the idle current would help that.

[Mouthurst]

Looks like your choke is overheating and leaking wax too. Lowering the idle current would help that.

Steve-thanks for the observation.
Well that choke is an old one and the wax was there from the begining.
Its heavy duty way more than standard ones for marshall or fender. But I will certainly keep an eye on it.
Im going to try the resistor change as you suggest.
thanks

[Mouthurst]

Ray

PS: Designing an amp to be easily serviced can tremendously increase the difficulty of construction; I know from personal experience.[/QUOTE]




Thanks Ray Ill keep the fan idea in mind although I dont think its not "enough"but I have seen some using them.
The "After Matchless 50" seems much better upside down mounted like the original.

Also I think a "standby" switch may help.

Well as far as designing goes its a hard call sometimes...I dont think there is anything inherently wrong with it I'll lower the idle and reduce the plate current and see how it goes.
I use it for recording only at about <half vol so It doesnt get too many hours a month-
The sound is the ultimate and woth all the headaches

PS--As far as the quote its from Leo Fender and goes "if something is easy to repair, it is easy to construct"
not giving much thought to it-
I merely added the bit about the "reverse not being true"

Thanks to both of you for your suggestions.
Time Marches On-

[Mouthurst]

Alex, yes I realize that but the AC30 OT is within the primary impedance range given, is about the same power, and when he said 'Class A' I didn't take him 100% literally (he could have meant "AC30 Class A" which we know really isn't Class A but...well you know). Two EL84s have about the same power as one EL34 so I don't see where the AC30 OT isn't suitable. What's the main reason it's not suitable?

Hi Matt-- heres a guide which may help-I cant vouch for its accuracy or its usefullness-
Amplifier Primary Impedance
Marshall, 50W 2xEL34 3,400 Ohms
Marshall 100W 4xEL34 1,750 Ohms
Marshall JTM45 2xKT66 8,000 Ohms
Vox 30W 4xEL84 4,000 Ohms
Vox 15W 2xEL84 8,000 Ohms
Fender 50W 2x6L6 4,100 Ohms
Fender 100W 4x6L6 2,000 Ohms
Tweed-Spec Cathode Bias 6L6 p-p 6,600 Ohms
Matchless 15W 2xEL84 4,000 Ohm
Matchless 30W 4xEL84 4,000 Ohms
Park 50W 2xEL34 5,000 Ohms

this should help the original poster as-well.

[cgiff]

El34 Bias level

Hi William,
Congratulations on getting your matchless amp up and running. I had exactly the same red plate problem. When I finally got the amp wired , I was lucky and it was playable first time I switched it on. I fell in love with the sound instantly , I played for about 5 minutes and then looked around at the output tubes and they were bright as the pilot light. Holy shit what's up. I had just bought Svetlana el34's .Yikes. Got on this website and a fellow amper, Dirty gave me some direction. Like you I do not want to compromise the tone. Back to the bench, 8 ohm load resistor power on. cathodes voltage was ~25 volts, what the hell. Measured the plate volts, It was high nudging 480V.
The screen volts was 465 - 470 v, all looked well. I calculated power dissipation by measuring across tube plate to cathode, calculated plate current Cathode v divided by resistance and yup tube power dissipation was way up. I bought 2, 500ohm (25W?) adjustable pots from Digikey and inserted into cathode of each el34. I started at 500 ohms did the voltage measurements, and speaker measurements. Used a signal generator, scope and voltmeters for each bias adjustment 500,480,460 ohms, etc. (I'm sorry I can't give you exact voltage measurements as this was over 4 years ago, I'm trying to be as descriptive as possible to the configuration and sound measurements as I Painstakingly did this) Excel spreadsheet on a laptop calculating power dissipation as I entered all measured values for each bias resistance change. The 4 pole switch allowed me to compare to the original bias configuration (for sonic comparison). It was a real mess but I did not want to compromise the tone. I think I got as low as 380 ohms before I got the plate dissipation lowered enough so the plates weren't incandescant, but still a tinge of red with the lights off. It's interesting to note that the tubes will find their quiescent operating point even though you add resistance your plate dissipation is still high if the plate voltage is too high. I originally had silicon diodes in the PS. I cloned the Chieftain circuit and somewhere I had picked up a schematic that someone had put voltage measurements on the schematic. My plate voltage was still too high still. I added a 5 volt filament transformer, a 5AR4 rectifier and this dropped the plate voltage about 35 vots I think I'm around ~430 PV if memory serves me correctly. This dropped the dissipation down further, the El34's are still on the edge of their dissipation, so be it.
William, you are going to have to attack your plate voltage. On my 2nd Chieftain I added a LARGE wattage power resistor to help drop the plate voltage . It is ~500 ish. I have 4 el34's in this amp so it's sucking some current The power resistor gets very warm, not smoking hot. I managed to drop the plate voltage to the 430 volt range with silicon diodes. Without the resistor the amp was red plating ( 4 X 380 ohm cathode resistors) not quite as loud but still chunky. There is no room to put in a pair of rectifier tubes without a sub chassis.The transformrs are so large, reverb pan etc in the head box No room period. Hope this helps. I wish I could be more technical but at this time there is no way for me to open the amp up on the operating table and get specific with actual values.
Cgiff.

[Gregg]

Recently I built an amp for a friend who was asking for class A/AB amplifier and I based it on the Chieftan output section which is by the way exctly as per EL34 data sheet. Luckily I had a custom toroidal PT which produced exactly the same voltages. Initially I was testing the amp with a pair of RFTs and there was no problem at all. Then I replaced them with a pair of EL34EHs and noticed that one of the tubes was developping red plate but this is only if you're looking for it. All voltages, currents and voltage drop on cathode resistors were the same. Then I swapped the tubes and the problem was solved.
I'm wondering why 10W cathode resistors are recommended provided that the actual power dissipation on those is ~2.5W. I'm using 5W resistors both for cathodes and screens.