6V6 Mystery Amp

[soulsonic]

Came across this mystery amp for cheap, so I snatched it up. Looks like an excellent candidate for a guitar amp project!

In some ways it seems like a typical 6V6 push-pull amp, maybe 15 watts? The 9 pin tubes were missing and aren't labeled, but the pinout matches correctly for the usual 12A*7 types. But there's a few strange things. Firstly, the amp has obviously been modified, at least with the addition of the RCA input jack; it doesn't match the rest at all. The bright yellow wires look newer than the others, they were maybe a repair? Strangest is the two small caps connecting the 6.3V heater supply to the input signal! I've never seen this ever in the many dozens of old amps I've been inside. Some kind of noise reduction?
The volume control is strange too: it appears to have two taps and an encapsulated network attached to them. My guess is this is a "loudness compensation" filter of some kind.
Other things: No fuse or power switch! Has TWO death caps connecting the mains to ground, instead of the typical one with a switch - twice the death!
The chassis hole that has the new RCA jack was originally labelled "Motor." No idea what this would have been originally, but it was clearly repurposed at some point.
Here's the schematic I traced:

Lots of feedback happening here. Each preamp stage is has local (degenerative) feedback, as well as feedback from the second back into the first stage, and global negative feedback. My guess is they were trying to get this thing as clean as possible; no fun for a guitar amp!
The phase inverter is a version of the paraphase style. It doesn't match well with any of the variations in the RCA book, so I'm curious about how well it balances. I'd like to replace it with a cathodyne phase splitter, but I'm up for trying this first to see how it behaves. Tone controls are typical for old cheap "hi-fi" style. Would rather just have a simple single knob tone for this.

[ppluis0]

Hi Martin,

Are you sure about the continuity of secondaries of the power transformer ?
Normally the 6.3v and the 5v for the rectifier heaters are fully isolated to other windings.
BTW the tone control are similar to those found in Ampeg amps. The potentiometers must be logarithmic in this controls.

Cheers,
Jose

[soulsonic]

Are you sure about the continuity of secondaries of the power transformer ?

You are correct. These are separate windings. I was being lazy with my drawing, not modifying the symbol to show the two separate windings.

[deltafred]

Those sort of amps were often fitted to the big old 50's and 60's radios and radio-grams, the "motor" would be in the record deck.

I used one very similar as my first bass amp (circa 1967), the bare chassis stood on a home built 2x8 speaker cab that housed 2 ex radio 8" speakers. It wasn't very loud but it got me started and I played my first 2 (admittedly low volume) gigs with it.

When the guitarist's parents bought him an AC30 it was very evident that it was time to upgrade!

[soulsonic]

Update on this:
The thing works, but has a bad distortion when turned up. The mystery network on the back of the volume control is indeed a loudness compensation filter, and it sounds awful.
The distortion is no doubt from the output tubes being wrongly biased. I measured the poor things were burning nearly 19w each! Not good for a 6V6.
At first I thought this might be a problem of leaky coupling caps putting positive voltage on their grids. The caps were indeed leaking a bit, but replacing them didn't solve the problem.
I started investigating the cathode bias resistor. It's value checks fine. The bypass cap checks fine. Disconnecting the bypass cap did not affect it either way. But, by trying different cathode resistor values, I was able to bring the idle current down to 10w of dissipation per tube. This seems more healthy.
BUT!!! The B+ voltage is very high. I'm getting nearly 430VDC on the plates. Not healthy for typical 6V6. I've gone through everything I could think to check, and it comes down to the fact that the HT secondary of the transformer is putting out 780VCT unloaded. This seems way too high for a 6V6 amp! The only reason the B+ is less than 480VDC is because of how inefficient the 5Y3 rectifier tube is.
I measured my mains voltage, and I'm getting almost exactly 117VAC. Not high at all. And the text on the chassis states it is expecting 117, so that isn't the problem. The secondary winding measures approximately 200ohm end-to-end, and almost exactly 100ohm between each end and the center tap. So, I don't think it's an issue of a winding having become shorted.

Could it be that this was really a 6L6 amp, and someone had put 6V6 in it? No markings anywhere to suggest one way or the other. Wish I had a pair on-hand to see how they bias with the stock cathode resistor value.
5Y3 rectifier and that small output transformer doesn't seem appropriate for a 6L6 amp, though. Maybe a low-powered 6L6 amp? But if that's the case, the voltage still seems high.

[ppluis0]

Hi Soulsonic,

Thinking that this amp module belongs to an old record player, perhaps the loudspeaker was of an electrodynamic type wich has a big coil instead an alnico magnet to develop the permanent magnetic field to work.

In this case, the current taken from power supply flows trough that coil and reduce the output voltage and also the ripple, as per the following diagram found on the web:


Try to add an suitable inductor in the same position and see what happen.

Cheers,
Jose

[soulsonic]

There is no evidence of this.

[lolbou]

The 6L6 hypothesis is likely... Check the output transformer for impedance?

[mozz]

Pair of 6l6s wouldn't really use a 5y3. Princeton's run 420v on the 6v6s so I wouldn't be worried. There might have been a separate tuner so extra tubes would draw more filament and plate current , that would bring the b+ down a little. What is your filament voltage now? Is it a older 5y3?

Edit, I'd put screen resistors in there. Get rid of those ac input death caps and those filament caps. I can't see overbiasing causing a lot of distortion. If your filter caps are original they are most likely bad.

[soulsonic]

Pair of 6l6s wouldn't really use a 5y3. Princeton's run 420v on the 6v6s so I wouldn't be worried. There might have been a separate tuner so extra tubes would draw more filament and plate current , that would bring the b+ down a little. What is your filament voltage now? Is it a older 5y3?

Edit, I'd put screen resistors in there. Get rid of those ac input death caps and those filament caps. I can't see overbiasing causing a lot of distortion. If your filter caps are original they are most likely bad.

I agree, 5Y3 doesn't seem like a good choice for a 6L6 amp. I only have old 5Y3. Never bought a new one of those in my whole life. There is a good chance that something else used to be powered from it that would cause it to sag down to a lower voltage. If it were around 350v, this cathode resistor value might make more sense and give a correct bias. But as this thing is now, I have to contend with this higher voltage. I can get it to bias correctly with 6V6 at 420v, but this requires changing the cathode resistor to over twice the original value. I just got some 6L6s, so I'll try those out with the original value resistor soon and see what happens, see if it does anything sane.
Filament voltage is almost right on the nose at 6.3v. I replaced the Hum Balance pot with a pair of 100 ohm resistors, very closely matched RN60 type
Surprisingly, the original electrolytic filter caps all seem fine. There is almost ZERO hum and noise in this amp. It's quite inexplicable. I've heard bad caps plenty of times before, and I'm not hearing it now.
The coupling caps are a different story. All the wax paper caps were leaky, so they've all been replaced. The death caps are gone, and now it has a three prong cord, fuse and power switch. I was hoping the leaky coupling caps were causing the bias issues, but that wasn't the case. I haven't listened to it yet with the new coupling caps, because I got caught up with figuring out the bias situation. It's been all meters and numbers the past few sessions with this amp. Should actually play through it and see if the new coupling caps fixed the bad distortion.

[soulsonic]

I need to stop being lazy and measure the impedance of this output transformer.

[Ben N]

430v on the plates & 19W dissipation -- sure sounds like a 6L6 amp. The fact that Fender ran 6V6s hot isn't really relevant here, IMHO--this is supposed to be an audio amp natively, not a guitar amp. Yup measure the tranny. Cool project.

[mozz]

If you do run 6l6, they will be drawing about 1 amp more filament current. So remeasure your 6.3vac, if it drops lower it was not meant for them, also see how hot the power trans gets after a while. I'd say a older amp would be a 8ohm or 16 ohm speaker so calculate the output trans from that. Just because it's dissapating 19w doesn't mean it's 6l6. I mentioned the 420v on a Princeton because the tubes can handle it. Manufacturers wanted to squeeze every watt they could out of a amp, wattage helps sales, they didn't care about tube life.

[soulsonic]

Update on this. I finally got around to trying some 6L6 types with this (got sidetracked with this sweet Traynor I've been fixing up and modding). To be exact, I tried KT66 in it. So, going with mozz's advice, I checked the filament voltages first to be sure the power transformer was OK with them - especially since the KT66 draws even more filament current than the 6L6 - and surprisingly, it seemed just fine! With 6V6 tubes, I measured filament of 6.7VAC; with the KT66 it was 6.46! Not much drop and still above the rated 6.3v, so it seems safe. Power transformer gets a bit warm, but not hot at all. Can still comfortably touch it, so I would say not hot, and less warm than some other amps I've had.
Now, here was another surprise, with the stock cathode resistor, the KT66 were dissipating 20w each. Not bad! The plate voltage measured at 361v with the KT66.
I wonder if the original tube set were metal 6L6? Could be old enough, and those seem like a good fit, physically, for this chassis, and if the biased up about the same, the dissipation would be consistent with their 25w rating (edit: oops, metal 6L6 is rated at only 19w dissipation). Another thing, the cathode bypass cap for the power circuit was included as part of the main multicap, and its section is only rated for 25v... Getting 32v dropped across the cathode resistor, so that's no good. Even with 6V6 it was around 30-something volts across it, so I disconnected it and replaced with a 40v cap. Explains why that cap can was getting hot!
Well, anyway... it seems to be pretty safe with these KT66, and now I can turn my attention to fooling with the rest of it. Still has splatty distortion when cranked, but I've learned enough recently from my experiments on the Traynor (and Merlin's excellent book), that this is easily overcome with making good choices in the preamp.
The 5Y3 seems underpowered for an amp with a pair of 6L6 types, and the output transformer looks anemic for anything over 20w. If this was a 6L6 amp, maybe it was one of those rated for around 30w? Anyway, I think the only way 6V6 would have been working correctly in this thing is if the B+ voltage was much lower, otherwise it would never bias correctly with the stock values. There are several random holes that look like they had wires passing through them originally. It's very possible that there was another chassis of additional circuitry - perhaps a radio tuner? - that was powered by this and dropped the B+ down to something that would be right for the 6V6... we'll probably never know. But in any case, it seems it's not a 6V6 amp anymore, so I'm just gonna work from there and get it tweaked out for these KT66s. Fun project!

[phatt]

Regards this;
Still has splatty distortion when cranked, but I've learned enough recently from my experiments on the Traynor (and Merlin's excellent book), that this is easily overcome with making good choices in the preamp.

With no grid stoppers it's a fair bet it will splatt
I'd be inserting Grid resistors on the power valves before tweaking preamp.
I went in circles on a couple of test builds until I worked that one out,,
One teck chap here just uses 10k on everything. worked for me
Phil.

[mozz]

I'm still guessing it's a 6v6 amp. Tubes look original and i don't think it's old enough that it used metal 6l6's, they are really old.
6.7vac is not bad considering it was probably meant to be operated on 110v or 117. Metal 6l6 would be 19w diss but i can;t see them using a 5y3 for any 6l6. A 5u4 is huge and would be very close to the trans and can cap, so i can't see that ever being used there. As for the KT66, if you do convert it to a guitar amp, i think you are gonna see major sag of your plate voltage when you hit a power chord.

I rip out tube amps from everything all weekend long and still stick with it's 6v6 amp. I've seen some biased really hot trying to sell it as a 25 or 30 watt amp. You have a variac?

[graeme]

Hi
You definitely need to put a couple of 1k screen resistors in to save your 6v6's. Fender used 470 ohms but that is not enough to protect the tubes. and another vote for putting in big grid stoppers on the power tubes like Phil suggested........

[soulsonic]

Thanks for the suggestions and insights everyone. I think I'm finished tinkering with this amp (at least for the time being!).
I've got it doing what I want now and I'm getting a satisfying tone from it. What I was looking for is something that can put out at least a few watts cleanly and then crunch just a bit when cranked; that kind of natural crunch that isn't too saturated or sustained.
Regarding the output tubes; I've found 6V6 give the best clean tone, but the KT66 give the better crunch tone. Could come down to them being biased to very different operating points. Still would like to tinker with not running the 6V6s so hot, but they seem to be taking it OK? They aren't redplating or seeming distressed otherwise - though they are noticeably less dynamic than the KT66 when pushed into overdrive. This could be just a characteristic of these very different tubes? In any case, I'm feeling more comfortable with running 6V6 in it now with the hot bias, especially since it's doing the clean tone I want. I'm going to get a new set of E-H 6V6s to run in there, so I don't have to worry about burning up my precious Sylvania blackplates (which is the main reason I worried about it being so hot).
It has grid stoppers and screen resistors now, and it doesn't splat anymore at all, even with the bass bumped up all the way. But I've also completely rewired pretty much everything.
Firstly, the phase inverter is now a cathodyne and the preceding stage is DC coupled to it thru a voltage divider. The input stage is now a cascode! First time trying this in an amp; sounds nice! I have a SPDT center off switch to toggle between 1uF, 47uF or no cathode bypass cap. Very useful ranges. The original tone controls have been left completely intact. I had planned to change them, but once I got the rest of the amp sounding good, I found them to be very satisfying and usable as-is. I swapped out the original tapped volume control because it had a very muffled tone at low settings, and then I realized this was likely because it was 3 Meg! So, that was replaced with a 1M and sounds much better. There's no bright cap on it; I generally don't like bright caps because I don't like how the tone changes so much as the volume is increased (I fought with this in old Fenders when I was a kid in high school until I later learned what was actually going on and stopped using them to "get my tone" at a low volume only to find it disappear when I turned up to jam with the band, etc...). The stock treble control actually has the perfect range to add some nice chime at whatever volume setting.
The negative feedback is connecting to the cathode of the phase splitter and the resistor has been changed from 15k to 470k. 15k with the 47k cathode resistor would have been giving way more feedback than it originally had and certainly more than I wanted, so I recalculated to something close. I tried it without any feedback, but it was a little too "angry" sounding for what I wanted.

I'll post up a revised schematic showing all my mods/fixes, soon.

[mozz]

I'd like to know what the 6v6 are running at plate voltage wise and also the kt66. Then how much are they sagging? Just guessing the kt66 can run at 550v and if you have them at 365, maybe that's why they are a bit crunchier. Sound good you have it running. I would have gutted it and made another 5e3 probably. Last old amp is now a 6v6 bassman in a Marshall style head, almost done except for biasing and nfb switch.

[soulsonic]

Here's where I'm currently at with it. This sounds great:

Has a nice clean tone that gives a nice mild crunch when cranked. Seems to respond very well with pedals.

Regarding voltages, here's what I measured:
HT Secondary, loaded with 6V6: 726VAC
With 6V6 and 5Y3 rectifier
Plate: 393v
Cathode: 25v with 330 Ohm cathode resistor

HT Secondary, loaded with KT66: 712VAC
With KT66 and 5Y3 rectifier
Plate: 367v
Cathode: 34.3v with 330 Ohm

Now, I thought I might try something different with the rectifier. I had a spare 5AR4, so I went ahead and tried that with the KT66, knowing it would have much less sag...
With KT66 and 5AR4 rectifier
Plate: 420v
Cathode: 39.5v with 330 Ohm

My conclusion is that I'm getting much more 6L6-friendly voltages with the 5AR4, but the bias is on the hot side (would be even hotter with the stock 300 Ohm cathode resistor), so I'm guessing that this likely was not the original setup either. And the output transformer really is too small not to get cooked by more than 20w in my estimation. I'm going to assume this is a 6V6 amp and that the B+ was originally sagged lower because of additional circuitry that is no longer there. The text on the chassis states it wants 117VAC input and my mains measures 116.8VAC, so this isn't a matter of B+ voltages getting too high because of my mains being high. But get this: next to the "117VAC" text, it also says "300W Max". So, whatever else was powered here could have been potentially drawing up to 300w in total? I don't see the power transformer being able to deliver that, so I'm going to assume this is also including things like a turntable motor that ran directly from the mains, but still, it's definitely pointing at more circuitry involved. Leaning heavily on thinking this powered a tuner as well.

Date code on the volume control is from 1957, if that gives you any clues about what would be normal for the time period.

Further conclusion: this amp sounds best overall with 6V6 as it is. It was fun to see how the KT66 sounded with it, and trying them at higher voltage with the 5AR4, but the 6V6s with the 5Y3 are getting the most satisfying tone to my ears. The KT66 have kind of this harsh sizzle going on in the top end that at first I thought sounded cool with the crunch, but now I find fatiguing. No big deal, because those KT66 sound awesome in my Traynor, so that's where they're going to live.

edit: fixed a goof in the schematic...