home brew = mystery amp

[Valvehead]

Someone brought me a home made tube amp to fix. No tubes in it . Needs a power tube socket . Its got 2 octals..pin 1 and 8 are tied then go through a 1 ohm to ground ....1K screen resistors...would this be for a EL34 then ?

if i remember pin 1 wouldnt be wired on a 6L6 type?

[Enzo]

Pin 1 doesn't NEED to be wired on a 6L6, but that need not stop one from wiring it up anyway, then most all the tube types can plug right in, as long as bias needs are met.

[J M Fahey]

Agree and add= measure and post bias voltage.

[nosaj]

Someone brought me a home made tube amp to fix. No tubes in it . Needs a power tube socket . Its got 2 octals..pin 1 and 8 are tied then go through a 1 ohm to ground ....1K screen resistors...would this be for a EL34 then ?

if i remember pin 1 wouldnt be wired on a 6L6 type?

Instead of guessing how about this. No tubes , power it up and show what voltages are on what pins. There are many octals out there.
nosaj

[Valvehead]

i will after i replace the socket----cheers

[Valvehead]

Ok---new socket --all wired up.

the 5y3 rect is putting out 566vdc

measuring the power tube sockets with no tubes =
pin 1 0v

2 and 7 have 6.9vac heater volts

3 has 566vdc

4 has 566vdc

5 has -14vdc
6 -14vdc (the bias pot can adjust it to - 64vdc )

8 ov

this thing has some juice !

[Enzo]

How about a nice clean, well lit photo of the underside of the chassis?

[Valvehead]

pics

[nosaj]

pics

Whole time I thought we were talking about a 2 tube amp based on original post.

nosaj

[Chuck H]

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.

[vintagekiki]

If pin 1 and 8 are tied this is the EL34 wiring.
1 ohm to ground is for cathode current measurement (bias adjustment)

[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.

ive got a mesa 5U4 here somewhere


what about 6550s / kt88 in this thing?

[nickb]

ive got a mesa 5U4 here somewhere


what about 6550s / kt88 in this thing?

The 5Y3 will drop something like 60V so knocking the B+ down quite a bit when loaded. Agree it seems like a poor choice for the expected DC current.

Another couple of clues would be the physical size of the output transformer and it's turns ratio. Does it look like a 25W or a 50W OPT?

You can figure the turns ratio by connecting the heater AC supply to the secondary (disconnect both wires to do the test) and measuring the heater supply VAC and the plate to plate VAC and taking the ratio.

[Justin Thomas]

Another rectifier alternative if you want to keep the high voltage drop is a 5R4. Takes higher AC voltage than a 5Y3 but still drops 50V when loaded properly. Has 2A 5V heaters, too so less stress on that winding.

Justin

[Valvehead]

the power trans is Huge...OT looks the size of a 50 or 100w . Both have solid black covers...no brand on them.

part number on PT is 54-13 138839

OT is 51-14 138826

edit... looks like they are Heathkit