Wonderful little homebuilt tube amp, would like to know what I have.

[altecchris]

I picked up this wonderfull homebuilt amp awhile back and would just like to get others opinions and specific info on what style tube amp it is and thoughts on it.

I unfortunately know very little on the history of this amp. I purchased it from a gentleman who had acquired it from the estate of the gentleman who had built it, per the gentleman's family he was respected tube guru before his passing, and this was one of his personal prized amps, and it shows in my opinion when you see the quality of the build he did on it.

I pulled the bottom cover off today for the first time to snap a couple shots for this thread, I expected it to be done well, but I dont think I have ever seen a wiring job done so meticulously, quite impressive and audio art to me.

Specifics

Tubes- two 12AU7, two JAN 6C4WA, four sylvania brown bottom (5831 only number I can find on them)

Transformers are hammond brand, two 1650 G input transformers,one 272JX power transformer.

Appreciate any info anyone can pass on thanks

 

[trinhsman]

Message Gordonw. He is a master on this type of thing.

 

[kward]

The output tubes are probably 5881's. It looks to be a push pull amp. Tube compliment could indicate a "Mullard" type circuit--6CW4 for the voltage amp and 12AU7 for the cathode coupled differential pair inverter/driver, with 5881's in push pull configuration for the output stage. If that's the actual circuit used, it's a decent performer.

Looks like the output stage is cathode biased, and it might be ultralinear wired, I can't quite tell.

The 272JX is a 600V center tapped type and the builder used silicon rectification. Plate voltage is therefore probably about 380V, and that should deliver 15 to 20 watts output, per channel.

Looks like the builder knew what he was doing. Nice find.

 

[thorpej]

Hammond 1650G is a 6.6K CT primary, which is a common plate load for push-pull 6L6-type tubes. So I agree with Kevin's assessment of the output stage. I'm not sure about his guess about the front-end, though... if you look closely at the belly shot, it sure looks to me like the outputs are being driven by the 6C4s (the two green non-twisted wires). If that's the case, I would have to assume it's configured as a cathodyne phase inverter (the tag board next to the 6C4's sockets look like they have 2 load resistors and 2 coupling caps, which is also further evidence of cathodyne'ness), and thus would not be a Mullard-style circuit.

You can also see that the the 12AU7 is direct-coupled to the 6C4 (wire from pin 6 of the 12AU7 socket going to the 6C4's tag board, and connected to a terminal that also holds the long lead of the 6C4's grid-stopper). The 12AU7's tag strips have a coupling cap on them, but the photo isn't clear enough for me to see any more detail in how those socket's are wired.

So, I'm going to venture a guess that it's 2 garden-variety 12AU7 gain stages, the second of which is DC-coupled to the 6C4 cathodyne phase inverter, driving push-pull 5881s. Like Kevin, I can't quite tell if the output stage is ultra-linear wired or not, but I'm going to guess that it is, because I don't see any evidence of a power supply node for the 5881 screens.

 

[gadget73]

I'm also going with the 6c4 as a split load inverter with the 12au7 as a gain stage. Physical layout agrees with that, as does the wiring.

Its a very nicely made amp, layout and parts selection is quite nice. For some reason it looks vaguely familiar though. Maybe someone on here had this amp or one like it in the past, dunno.

 

[sKiZo]

Nicely done. Unusual setup on the speaker connections. I'm assuming it's 4-8-16 left to right?

 

[thorpej]

Nicely done. Unusual setup on the speaker connections. I'm assuming it's 4-8-16 left to right?

Presumably to make it easy to use one of those standard 2-prong banana plugs for any of the outputs.

 

[altecchris]

Thanks guys for the info, gadget 73 the prior owner I purchased it from is a member on here, I wouldn't be surprised if he had posted about it several years back when he acquired it from the gentleman's estate who built it, he had told me when I bought it that he had been curious about its design.

I have had it ro two years now. I have been running it on my Bozak symphony speakers, this amp has a very open sound with a tad harder presentation than my audioknote el34 single ended tube mono blocks.

sKiZo, yes it does have 4,8,16 ohm speaker taps, I am wanting to try the 16 ohm tap on my heathkit legato hh-1-c, they have a 16 ohm tap as well as a standard tap, I've read it can give you better damping using the 16 ohm tap on an amp like this with 16 ohm speakers, I'm sorta a tube novice so what can I expect from doing this?

Also before I run it on my legatos I need to sort out an intermittent tube socket, it's on one of the 12AU7 tube sockets, I have to push the tube to one side sometimes for that channel to come in, it can cause quite a spike and not something I want to subject my legatos ro, all the solder joints on the bottom side look solid, is replacing the socket the best or is their a trick addressing this issue.

 

[kward]

Ah yes, the 12AU7s are in front. Good call

 

[thorpej]

Also before I run it on my legatos I need to sort out an intermittent tube socket, it's on one of the 12AU7 tube sockets, I have to push the tube to one side sometimes for that channel to come in, it can cause quite a spike and not something I want to subject my legatos ro, all the solder joints on the bottom side look solid, is replacing the socket the best or is their a trick addressing this issue.

I'd guess that one of the metal sleeve that grips the tube pin is a bit stretched. I've never re-tensioned socket pin sleeves before, and I don't know how feasible it is on 9-pin miniature sockets...

 

[sKiZo]

The damping thing only comes into play if your speakers have multiple drivers that are wired in parallel. IF not, the only real difference between running 8ohm or 16 ohm on matched tap components is that you can run longer leads at 16 ohm without signal loss. Not much of a problem with the usual home system.

With the intermittent tube, I'd think it's more of a dirty pin problem, or cold solder joint. Noval sockets don't normally need to be adjusted. Dental brushes and contact cleaner work nice for the sockets ...

And standing the pins in cleaner or alcohol for a bit and then buffing them with a q-tip works well.

 

[altecchris]

damping thing only comes into play if your speakers have multiple drivers that are wired in parallel. IF not, the only real difference between running 8ohm or 16 ohm on matched tap components is that you can run longer leads at 16 ohm without signal loss. Not much of a problem with the usual home system.

the legatos have a dedicated 16 ohm tap and one for 8ohm amps, they use two 15" 803 altec woofers per cabinet, I'm wondering if that's why they have the 16 ohm tap

 

[electronjohn]

That's a purty one there! As far as an intermittent socket goes...I've been able to stick a HEAVY sewing needle alongside the sockets and work them a little tighter. Worth a try before the hassle of installing new sockets.

 

[altecchris]

That's a purty one there! As far as an intermittent socket goes...I've been able to stick a HEAVY sewing needle alongside the sockets and work them a little tighter. Worth a try before the hassle of installing new sockets.

Thanks, I'll have to give that a shot with using the needle

 

[jkaley]

Thanks, I'll have to give that a shot with using the needle

Drain those caps before you go poking a needle in those tube sockets.

 

[altecchris]

Drain those caps before you go poking a needle in those tube sockets.[/QUOTE

I was wondering about that. Gonna have to Google that

 

[primosounds]

the possibility of getting shocked is quite real,.especially if you do that pin cleaning soon after the amp was on. Also if you have very large power supply caps without a drain resistor.