home brew = mystery amp

[mozz]

http://akdatabase.com/AKview/albums/...ansformers.pdf

[Valvehead]

At 566V unloaded there's no way that's for a pair of small bottles. So, that 5y3 rectifier is unsuitable. I don't know how much spec you can decipher from the power transformer, but if it "looks" like it could handle a pair of big bottles then the amp is probably for a pair of big bottles. Regarding the unloaded filament voltage, at 6.9V I wouldn't expect a pair of el34's. By my experience most unloaded filament voltages would need to start higher than that to manage the current requirements of el34's. That isn't to say the builder didn't make some errors in component choices so anythings possible. Still, I'm going to guess a pair of 6l6 types and a 5u4 rectifier.

54-13

http://akdatabase.com/AKview/albums/...ansformers.pdf

[Valvehead]

I tried a 5U4 , it put out 582V.

what tube would work here ?

[Chuck H]

Ok... Since the amp is wired with pins 1 and 8 connected, has the ubiquitous 1k screen grid resistors and the loaded plate voltage will likely be somewhere around 500V I'd say the builder intended el34 tubes. There are two hitches here though. Since a modern guitar amp is typically built to be cranked without risk I'd say the 134mA rating on the HV secondary is only marginally capable without overheating in the short term. Then there's the 4A filament rating which is, as I guessed earlier, too low for a pair of el34's plus four preamp tubes (averaging .75A each) which would put the current through the filaments at 6A. So we have an 810V CT with a 115V primary (essentially making it an 850V CT plugged into a modern AC outlet). 850V CT @ 134mA !?! What the hell tubes was that PT made for in the first place? IMHO it's not really suitable for a guitar amp without risks and/or concessions. JM2C on that. Of course I'll be "corrected". But the truth is that if you run the amp near full power much with el34 tubes AND at that filament wind rating then the PT is probably going to overheat. If you use 6L6 type tubes you'll still be over the filament wind rating, but probably not dangerously. But most modern 6L6's aren't happy at 500V. Most guitar amp PT's for a pair of big bottles start around 200mA HV rating. 500Vp is just too high for small bottles. Point is, I can't find a tube for that PT that isn't problematic in some way under guitar amp circumstances.

Unfortunately the OT is listed in the link, but has no data specs indicated.

[Enzo]

0.75A each? I thought 12AX7 6v wired drew 0.3A.

[Valvehead]

might be time to shop for new PT

[vintagekiki]

The question is whether is worth the investment (new PT. No tubes in it . Needs a power tube socket ...)

[eschertron]

I tried a 5U4 , it put out 582V.

what tube would work here ?

All your measurements have been with no power tubes in, correct?
Can you set up some resistors to draw around 130ma from the rectifier? I'm thinking to see the working voltage, we need to have a working load.

[Chuck H]

0.75A each? I thought 12AX7 6v wired drew 0.3A.

Your right. I didn't check that.

So that would be 4.2A on the 4A rated filament winding with el34's. Which is probably alright.?. A little light. As is the 134mA rating on the HV winding. Which is probably also alright.?.

Try it out. See how it sounds. If the PT gets stupid hot you'll know you have a problem.

[mozz]

Put a pair of el34s in there and see what voltages you end up with. Other than that, the power transformer/transformers may be worth more to someone looking to restore the Heathkit W5m

[Chuck H]

Like mozz I did a little digging to find what the 54-13 was used for. Typically mono block type amps that are never intended to be clipped. Also, the rectifier of choice seems to be the 5R4 as Justin studiously suggested. A 5U4 would probably be ok too, but won't drop quite as much voltage.

Anyway... At 134mA for the HV winding at roughly 500Vp you can reason that a pair of el34's alone will draw 100mA just on the plates at full power and a a good bit more when clipping. Then we need to add the screen draw, losses in the transformers, power supply circuit losses and preamp circuit draw. I don't have approximations for these, but it's significant. That said...

Nothing to lose. I know that in my experience it's VERY hard to hurt a Hammond transformer stressing it's max spec. So maybe this transformer is equally robust. Give it a try and see.

[Valvehead]

The question is whether is worth the investment (new PT. No tubes in it . Needs a power tube socket ...)

i will have to ask the owner

[mozz]

From looking at the pictures, it looks like a pretty decent homemade build. I would say well worth getting it running, i doubt very much it needs a power transformer, you just need the right tubes in there. El34 in there i can guess it's gonna drop a lot closer to 500v or less. Start drawing the schematic.

Why there is a mix of old parts (blue fender caps) and newer parts is still puzzling.

[vintagekiki]

Why there is a mix of old parts (blue fender caps) and newer parts is still puzzling.

In home made, mostly embeds parts that are handy on the table, or in the bin.

[Tony Bones]

Those transformer part numbers suggest that they came from a Heathkit A9 which was PP 6L6, ~20W. But that does NOT mean that this homebrew amp was meant to take 6L6's.

The L shaped board looks like it might be a bias supply. With the amp powered up (no tubes) probe pin 5 of either power tube socket while twisting the knob on the L shaped board. What range of voltages do you see? This will help decide what tubes might work.