Small Tube Amplifier Conversion

[JKowalski]

Alright, I got my hands on a super cheap tube amplifier at a thrift store. It was a stereo amplifier for a record player, super simple. Two EL84's, one 6au7, a rectifier tube, two OT's (for 3 ohm speakers.... weird), and a PT. DC voltage on the caps is 254V.


I just scrapped the wiring inside yesterday. I want to build a small tube amp, el84 PP. I want to salvage the PT as well.

My question is, is there any good amps that you know of that use 250V, with two el84's P-P? I could end of designing a new one, but I just want to know my options. I've done my research, so don't think im trying to freeload here, I just want to make sure I didn't miss anything great. I'd prefer to use the preamp tubes that I have at my disposal at the moment - which are 12ax7's, 12au7's, and 6x8's. The last criteria is that I want to try to get as much power as possible out of this.

All suggestions are welcome, however.

Also, what's the best way to estimate the power ratings of my PT? \




PS Sorry I don't have a pic, I'll have one up tomorrow. Don't have a camera just at the moment. It's a tiny little thing, about 9x5". Gonna have to use point to point wiring, I guess, to salvage the chassis. There's plenty of posts inside from the previous circuit, so that's not a problem.

PSS If you are wondering about my electronics background, it's pretty good. I am very experienced with solid state electronics, but only a slight amount of tube knowledge. This will be my introduction to tube design. And don't worry if you think designing an tube amp with minimal experience is out of the question for me, because I know what I am doing here as far as safety, and I have a ton of patience and a big desire to learn. If it takes a year to finish this thing due to all the learning I have to do along the way, that's fine with me.



PSSS (haha) I also have a tube organ I got at another thrift store for $0.80. The lower key register doesn't work, or works only sporadically, and I am considering scrapping it for parts. 54 tubes inside in all. The PT is enormous (all those heaters!) and the output tubes are 6V6's. Do you think it is worth it, to scrap it? I bought it orginally to do just that, but I kind of like the sound and wouldn't mind having this thing in my room. It shouldn't be a hard fix, cause I think the only problem for the lower keys is that the voicing switches are messed up.

Messing around with that organ also got me my first mains shock... It has an interlock switch on the back, with two prongs that insert in when you close the back. I removed the prongs from the cover and stuck them in, so I could see what state everything was in as I turned the organ on. When I was playing the organ for a while, one of the prongs slowly wiggled out and was stuck diagonally in the interlock. Somehow - this caused the on/off switch for the organ to have mains on it???? Yeah - not fun. At least it only lasted for the second that it took to finish the switching off motion....

[olddawg]

Use the power transformer and one output transformer for a mono amp. Since the PT voltage is low for EL84s, convert it to a SS bridge rectifier. That will bring the B+ up and increase the output power more in line for an EL84 guitar amp. My suggestion is to build an 18 watt Marshall clone like a Lite IIb. If you don't know what that is go to 18 Watt Community Center and check it out and other variants. IMO the 18 watt Marshall is one of the best EL84 amps. I would suggest using a fresh chassis, new tube sockets, and new pots. Also, I would suggest using a turrent board architecture which is easy to build, mod, and experiment with. You could have a nice 18 watt Marshall clone head for around $100 minus the box.

[JKowalski]

Use the power transformer and one output transformer for a mono amp. Since the PT voltage is low for EL84s, convert it to a SS bridge rectifier. That will bring the B+ up and increase the output power more in line for an EL84 guitar amp. My suggestion is to build an 18 watt Marshall clone like a Lite IIb. If you don't know what that is go to 18 Watt Community Center and check it out and other variants. IMO the 18 watt Marshall is one of the best EL84 amps. I would suggest using a fresh chassis, new tube sockets, and new pots. Also, I would suggest using a turrent board architecture which is easy to build, mod, and experiment with. You could have a nice 18 watt Marshall clone head for around $100 minus the box.

I didn't really want to get an account there at the moment (lazy & busy : P) but I found this marshall 18 watt amp on schematic heaven (1974). Is this the same one?

http://www.schematicheaven.com/marsh...watt_schem.pdf

I really wanted to keep the chassis for this project - I want to make this project as cheap as possible, under $100! I wanted to make use of the tubes that I currently have in my collection, the power transformer, and the chassis - the can cap I was planning on scooping out and installing new electrolytics inside, and I am probably going to wind my own output transformer.

The only things I was wanting to buy really were new potentiometers, the electrolytics, and whatever power resistors I need. Depending on how expensive, I might decide to splurge a bit on an OT instead of DIYing.

Your SS rectifier is a good idea, thanks alot. If I am using this chassis, it frees up a pre-drilled tube socket as well, leaving me with four nine pin sockets.



EDIT: I just cleaned out the chassis, de-riveted the OT's, and steel wooled the whole thing, and it looks really nice - but the main reason I added this to the end is that there is totally enough space to add a small turret board! If I remove the terminal strips, there is a nice 4 1/2" x 2 1/2" free spot. I could mount the OT above the turret board, as well. I know it's small, but still. More options.

[olddawg]

The 1974 is what all of the 18 watt Marshals are based on. The trem channel is quirky though. The variant I mentioned is single channel, has more gain, is much easier and inexpensive to build, uses fewer parts, and needs less space. You can mount the SS rectifier and the filter caps on the same turrent board. If you don't use a tube rectifier that will also leave you with more filament current available for the preamp and PI tubes. Seriously, if you plan to build an 18 watt take the one minute it takes to register at 18 watt.com. The process is only to keep spam of the board. Lots of schematics, advice , and expertise are there for free. They've been building Marshall clones for ten years. BTW, I have an old Zenith OT in my last 18 watt build and it sounds great. The original OTs were just off the shelf hi fi OTs anyway. Some people make a big deal about having "authentic" transformers. I have had great results reusing old OTs from hi fi amps.

[JKowalski]

The 1974 is what all of the 18 watt Marshals are based on. The trem channel is quirky though. The variant I mentioned is single channel, has more gain, is much easier and inexpensive to build, uses fewer parts, and needs less space. You can mount the SS rectifier and the filter caps on the same turrent board. If you don't use a tube rectifier that will also leave you with more filament current available for the preamp and PI tubes. Seriously, if you plan to build an 18 watt take the one minute it takes to register at 18 watt.com. The process is only to keep spam of the board. Lots of schematics, advice , and expertise are there for free. They've been building Marshall clones for ten years. BTW, I have an old Zenith OT in my last 18 watt build and it sounds great. The original OTs were just off the shelf hi fi OTs anyway. Some people make a big deal about having "authentic" transformers. I have had great results reusing old OTs from hi fi amps.

Don't worry, I planned on registering eventually - today's a sort of clean up day in my "workshop" so I am kind of busy - I just left this topic going so I could have something to go off of at the end of the day

I just mentioned the schematic heaven one cause I wanted a quick look at the basic structure of the amp.



As for the OT, I still need to get it from somewhere... The OT's that I have are for single ended amps, so they ain't really compatible. I didn't mean I would spend alot. I kind of meant "splurge" as in "the only thing over $15" - of course, unless I rewind something.

Thanks alot for the replies, by the way.

[olddawg]

You have another issue as well. The reason I suggested that you use an SS rectifier to bring up the B+ is because the PT HV is so low. The original 1974 had a 300-0-300vac secondary. For a clone with an EZ81 rectifier tube most people prefer a 290-0-290 nowadays because line voltage is higher. In the Lite variants people use a 275-0-275 because of the SS rectifier. In any event, the magic B+ for E84 guitar amps seems to be about 350vdc by consensus. A 250-0-250 will give you give you less output watts and slightly different characteristics. You may or may not like it. It is however done all of the time.

[unparalleled]

Don't worry, I planned on registering eventually - today's a sort of clean up day in my "workshop" so I am kind of busy - I just left this topic going so I could have something to go off of at the end of the day

I just mentioned the schematic heaven one cause I wanted a quick look at the basic structure of the amp.



As for the OT, I still need to get it from somewhere... The OT's that I have are for single ended amps, so they ain't really compatible. I didn't mean I would spend alot. I kind of meant "splurge" as in "the only thing over $15" - of course, unless I rewind something.

Thanks alot for the replies, by the way.

Heaps of circuits here:
The Blue Guitar
Check out "Trainwreck" schematics.

[tedmich]

doesn't get much simpler than this:

or this


but first give the actual AC reading off the PT secondary so we know what you are working with; IME EL84 PP sound best at 320 or 400VDC B+

[JKowalski]

I checked earlier today after I stripped the chassis more thoroughly, the transformer secondary is 250V-0-250V. Based on my calculations of full wave bridge rectifier output, wouldn't that land the raw dc value near 450VDC? That sounds good.



Tedmich, that schematic is great. Exactly what I was looking for, now that I think of it - completely minimal, and easy to modify. I listened to a few Marshall 18 watt's on youtube today, and that just reinforced this decision. I'll take a peek into the 18 watt page tomorrow, and see what people have done, and use that as a springboard for what I decide to do.

Four tubes, as well, perfect fit. (unless I stick another in, of course)


Thanks so much, everyone!

[Wakculloch]

Remember that if you use a bridge circuit you're effectively doubling the current that needs to be delivered by those windings, and if you're doing that to increase the voltage, and more valves are involved there comes a point where the ability of the transformer to deliver the extra power will be exceeded and it'll get hot and all your voltages will be down.
If the transformer is physically large this may not happen, but don't expect a wee one to cope well if you push it.
EL84's work perfectly well with 250 to 300 volts on them, are more reliable and you wont be building a valve eating monster.

[JKowalski]

Remember that if you use a bridge circuit you're effectively doubling the current that needs to be delivered by those windings, and if you're doing that to increase the voltage, and more valves are involved there comes a point where the ability of the transformer to deliver the extra power will be exceeded and it'll get hot and all your voltages will be down.
If the transformer is physically large this may not happen, but don't expect a wee one to cope well if you push it.
EL84's work perfectly well with 250 to 300 volts on them, are more reliable and you wont be building a valve eating monster.

Yeah, I am kind of worried about that. The transformer is pretty small, that's why I was asking if there was an easy way to estimate the power capabilities of a transformer. The question still stands

If I use a traditional full wave rectifier, that's only 225VDC or so. Compared to the preferred 350, 125V off is quite alot...


I suppose I will figure it out.

Just need that power rating to start with...



EDIT: I just noticed, on the minimalist 18watt schematic, that the transformer secondary is 215VAC??? Yet they show the rectified DC voltage being 305VDC. Isn't that the peak DC value, not the true value....? Also, is this the measurement used when you say that the range of DC should be 300-350? If so, isn't that perfectly fine, with a 250VAC traditional full wave rectifier?

[olddawg]

Yeah, I am kind of worried about that. The transformer is pretty small, that's why I was asking if there was an easy way to estimate the power capabilities of a transformer. The question still stands
If I use a traditional full wave rectifier, that's only 225VDC or so. Compared to the preferred 350, 125V off is quite alot...
I suppose I will figure it out.
Just need that power rating to start with...

EDIT: I just noticed, on the minimalist 18watt schematic, that the transformer secondary is 215VAC??? Yet they show the rectified DC voltage being 305VDC. Isn't that the peak DC value, not the true value....? Also, is this the measurement used when you say that the range of DC should be 300-350? If so, isn't that perfectly fine, with a 250VAC traditional full wave rectifier?

A rule of thumb I see used a lot is to take the half wave peak secondary AC value and multiply it by 1.3. That will give you approximately what the filtered and rectified B+ will be with the bridge and caps. 250 X 1.3 = 325 vdc

[JKowalski]

A rule of thumb I see used a lot is to take the half wave peak secondary AC value and multiply it by 1.3. That will give you approximately what the filtered and rectified B+ will be with the bridge and caps. 250 X 1.3 = 325 vdc

Now I am totally confused. This is how I learned how to do it:

Bridge Full Wave:

Vrms x 1.414 x 0.637 = av. DC value

500Vrms x 1.414 x 0.637 = 450VDC

Center Tapped Full Wave

Vrms(1/2) x 1.414 x 0.637 = av. DC value

250Vrms x 1.414 x 0.637 = 225VDC

And of course, the center tapped is half as high as the full wave. I always used these formulas to find the DC value, and it has always been close to the filtered DC value for me. (at least av. voltmeter readings)

I see where the 1.3 comes from, I guess, as that's what I use for RMS to peak, but you said you multiply the peak value by 1.3, not rms?? Was that a typo?

I suppose you don't really count the 0.637 average DC value in this "rule of thumb", only peak DC?

[olddawg]

Now I am totally confused. This is how I learned how to do it:

Bridge Full Wave:

Vrms x 1.414 x 0.637 = av. DC value

500Vrms x 1.414 x 0.637 = 450VDC

Center Tapped Full Wave

Vrms(1/2) x 1.414 x 0.637 = av. DC value

250Vrms x 1.414 x 0.637 = 225VDC

And of course, the center tapped is half as high as the full wave. I always used these formulas to find the DC value, and it has always been close to the filtered DC value for me. (at least av. voltmeter readings)

I see where the 1.3 comes from, I guess, as that's what I use for RMS to peak, but you said you multiply the peak value by 1.3, not rms?? Was that a typo?

I suppose you don't really count the 0.637 average DC value in this "rule of thumb", only peak DC?

There was a hugh discussion on PT specs a while back on another site. Everyone was trying to figure out what the numbers meant. Is 250-0-250 500v peak to peak? Is it average? Is it RMS. Depending on the manufacturer it can be different things. Since you have it in front of you you can measure it on a scope for peak to peak (A meter will probably give you RMS) and then do your math. That's what I would do. A fellow in the discussion said that this 1.3 value will give you a ballpark estimate for a full wave bridge and filter using the commonly labled transformer specs. I find it to be pretty much correct. I misunderstood that you have stereo single ended amps. I thought you meant that you had stereo push/pulls with 4 EL84 tubes. Your PT may not have enough current (you generally need 100-120 ma) for 18 watts and the OTs will not work. It's probably a better candidate for a 5 watt single ended amp like a VJ.

[JKowalski]

There was a hugh discussion on PT specs a while back on another site. Everyone was trying to figure out what the numbers meant. Is 250-0-250 500v peak to peak? Is it average? Is it RMS. Depending on the manufacturer it can be different things. Since you have it in front of you you can measure it on a scope for peak to peak (A meter will probably give you RMS) and then do your math. That's what I would do. A fellow in the discussion said that this 1.3 value will give you a ballpark estimate for a full wave bridge and filter using the commonly labled transformer specs. I find it to be pretty much correct. I misunderstood that you have stereo single ended amps. I thought you meant that you had stereo push/pulls with 4 EL84 tubes. Your PT may not have enough current (you generally need 100-120 ma) for 18 watts and the OTs will not work. It's probably a better candidate for a 5 watt single ended amp like a VJ.

My PT is 250V-0-250V RMS. I wasn't going off of manufacturing specs, since they are impossible to find, so I measured it on the voltmeter.

I've already been resigned to the fact that the OT's aren't usable.

So, I need a transformer with about a 30VA minimum rating, correct? If I can manage to squeeze by with that much for the transformer, I'd be fine with using a separate transformer for the heaters.

Again, is there a way to estimate power capabilities? Maybe a size/weight comparison? The lamination dimensions are 2.25"x2.75"x1.25", if that helps.

Here's the pics of the stripped chassis. I might keep the two middle tube sockets, but the ones of the side I am probably going to replace.






If I can't get higher than 10 watts, I'll probably scrap this idea and make a tube preamp project with it instead. I'm really hoping I can manage to do this, though.

[tedmich]

just need ~30mA per el84, plus 7mA per 12ax7 section. Also if you don't dime it all the time you can go with a small PT. Whats wrong with the OTs? I didn't catch their spec...

EL84s can use 6-10k primaries PP or SE and output 4-8ohm.

Never buy new if you have usable iron!

[UniPlank84's]

Maybe this would work???
As said above why waste money and time converting to PP? If both the SE OT's are good I'd be making a Mini-z, Vjr, or Smokin'Joe into 2x outputs. 1x6-10" and 1x8-12" speaker or whatever would be a cool possibility. You may have great SE OT's right there-would just need passive parts and 2 speakers. I'd do a Mini-z circuit myself if I had that. Good find by the way looks in great condition.

[JKowalski]

just need ~30mA per el84, plus 7mA per 12ax7 section. Also if you don't dime it all the time you can go with a small PT. Whats wrong with the OTs? I didn't catch their spec...

EL84s can use 6-10k primaries PP or SE and output 4-8ohm.

Never buy new if you have usable iron!

Impedance ratio is 1600:1, 12.8k at 8 ohm, 6.4k at 4 ohm. Could I use two SE output transformers in a push-pull arrangement, by connecting the windings to each other for center taps?

[JKowalski]

Alright, well I managed to find a 15-20 watt 8k primary PP on eBay for around $10. It's new in box, and looks perfect for this.

So it's the 18 watt for me.

[unparalleled]

Alright, well I managed to find a 15-20 watt 8k primary PP on eBay for around $10. It's new in box, and looks perfect for this.

So it's the 18 watt for me.

The parts and chassis you have would be perfect for a Vox AC4.
In you case you would substitute silicon diodes for th tube rectifier.
To guitarists this amp's sound is very desirable and with an efficient speaker quite loud!
I have built them.
If you don't want Tremolo just leave out the ECC83. You could also use the input stage from the "minimalist" if you don't want to use EF86,
and you could also add an extra gain stage later with a full tone stack.
As far as power transformer voltages go, connect a rectifier to the darn thing with a 400v or higher rated capacitor connected across +/- and let your meter do the maths!
Just be sure to parallel a bleed resistor across the capacitor so it will discharge when you switch off.
Try 100Kohm for this and leave meter connected, you can watch the voltage fall when switched off.
A handy device to make before you start is a test lead with a 240v lamp socket in series with the phase wire and a power socket to plug in any amp you are testing.
If there is anything drawing excessive current the lamp will light up and limit the flow of current to a level which will save any smoke from escaping!
The wattage of the lamp used will set the limit and you can change bulbs to suit the amp under initial test.

[JKowalski]

I'm still going with a PP 18 watt. I just got the OT today. Picked up a step drill to enlarge the holes on the tube sockets and place some ceramic ones in.


I'm actually going to try a hybrid, with the 18 watt power amp and a preamp with ideas borrowed from other amplifiers. I am also thinking about adding reverb.... The chassis has two RCA jacks on top already installed, and the single ended OT's that were on the amp originally seem good for driving a tank (though a little overkill).

Tomorrow I am going to gut the filter cap can from the original and put new ones inside, as well as some 0.1uF parallels. Right now I am thinking about three 56uF 420V (they fit perfectly, according to my calcs) I may even put in a bleeder resistor (thoughts?). The main reason I didn't want to put them inside the chassis is obvious - there isn't alot of space. Just enough to fit everything in decently. I really like the size of the amp, my only concern is noise... We'll see how that turns out...

[olddawg]

I'm actually going to try a hybrid, with the 18 watt power amp and a preamp with ideas borrowed from other amplifiers. I am also thinking about adding reverb.... The chassis has two RCA jacks on top already installed, and the single ended OT's that were on the amp originally seem good for driving a tank (though a little overkill).

Although adding an onboard spring reverb has been done for 18watt EL84 type amps, the general consensus is that it doesn't work well. The magic in these amps is the way they break up. You end up running a distorted signal to the reverb driver with some pretty ugly results. Most people would tell you just to use a pedal and don't waste your time. The Bugera V22 uses a digital reverb in it's otherwise tube build, but I haven't been able to locate a schematic for one yet.

[JKowalski]

Although adding an onboard spring reverb has been done for 18watt EL84 type amps, the general consensus is that it doesn't work well. The magic in these amps is the way they break up. You end up running a distorted signal to the reverb driver with some pretty ugly results. Most people would tell you just to use a pedal and don't waste your time. The Bugera V22 uses a digital reverb in it's otherwise tube build, but I haven't been able to locate a schematic for one yet.

Yeah, I've read that, but I figured that I'd be redesigning the preamp anyways and I would do so to consider the reverb as well. Regardless, It's a no on the reverb. I have a pioneer tube reverb that would mesh fine with this, and I completely ran out of space after I decided to get a seperate heater transformer.

It's coming together, I just gutted that cap (that was a pain...) from the bottom, and installed all the new sockets and the transformers.

[JKowalski]

Here's what I came up with. It's the basic lite preamp, with an added gain/CF stage.



I read this article:

The Valve Wizard

And It looked interesting (I recently made a SS pedal design that let you get that kind of half-wave smooth clipping behavior, and liked the sound) so I incorporated it. I suppose I would have to see how the variable aspect works IRL to decide whether to keep it or not.

Thoughts on the design? Anywhere I completely screwed up?

Thanks in advance