Refurbishing some Ampex el34 monoblocks

How about the big 7W resistors? Can I go 10watt instead? I’m thinking yes, but just want to make sure before spending the cash.
 
I rebuilt some heathkit W4-am’s with the help of DerekV and Dave Gillespie and used some Russian PIO K40Y-9’s to good effect. I will try the K42Y-2’s I ordered and see how things go.
 
The pictures are worth a few thousand words!

The 6L6 outputs could be original in your amplifier. Ampex used the exact same chassis on their 6L6GC amplifiers. The 6L6GC’s are tall bottles like the EL 34’s -- so most of these look much the same in photos. Both of these are likely 1961 models. You might check for differences in the power transformers as well. The 1961 EL34 amplifier used an Ampex p/n 658-0025. I believe the 6L6 amplifier operated at lower power supply voltages. I haven’t yet encountered a schematic for the 6L6 version of this amplifier.

I can see that yours has had quite a few components replaced. What appears to be extra components is primarily under chassis replacements for the can capacitor. The work is clean. I would troubleshoot it and get it working before considering tearing it down. If they sound great, use the effort saved to repaint the chassis and transformers.

I would start with that schematic for the 40W amplifier and draw out what you have, making note of what appears to be original -- that might shine some light on whether it was originally built as a 6L6 amplifier.

The 6L6’s and EL34's are pin-wise interchangeable in some circuits but not in others. The difference is that the 6L6G has the third control grid tied to the cathode (pin 8) – leaving pin 1 is unused -- whereas the EL34 has the third control grid tied to pin one. The schematic for the 40W amplifier shows that pin 1 and pin 8 are jumpered at the socket -- so at least in terms of pin compatibility that chassis can take either tube. So look for a missing connection between pins 1 and 8 -- then you know it was a 6L6 amplifier. I would also look for any differences in components around the output tubes as well as any differences in the resistor value used in the feedback loop and any of the small compensation capacitors near the input amplifier tube.

The output transformer part number may suggest that it was originally an EL34 amplifier. The 6L6 amplifiers that I have seen built on this chassis use the Ampex by Triad p/n 58-0009 transformers -- which are the same transformers specified for the 1959 35W EL34 amplifier. Ampex was always revising parts so sometimes it can be hard to tell what was original.

Here’s a caution: When drawing out the schematic you’ll need to be very careful in comparing the physical wiring of the two power supply capacitors between your amplifier and your buddy’s…
When Ampex ended up with those two physically very long Mallory capacitors they weren’t able to use the original installation locations like on your buddy’s amplifier. Although both amplifiers have the same terminal strips in the same locations (save for those which were added to yours), three of the lugs have been reassigned. They were reassigned in a circular fashion. (A to B, B to C, C to A). Until you realize that those lugs were reassigned in a circular fashion this can be very confusing. I’m sure this will save you some frustration.

Of course you’ll notice that the ground point for the capacitors is now way over at by the input amplifier. If you replace those capacitors with the physically smaller capacitors available today, you might want to move that ground point back over near the power transformer more like the way your buddy’s amplifier is wired.

Q. Which was the capacitor that was improperly connected?
 
Man, audio projects often grind slow! Since I posted this original thread I did a full backyard remodel on my home, then I bought a rental home, remodeled it and began an Airbnb. Busy, busy, busy. I completely rebuilt one unit and then got caught up in the rest of those things. I’m about finished with the second unit and will upload pictures in the next couple days.
 
Alright. So I did a complete rebuild. The only thing that is original is the iron, the fuse holders, and the 470 ohm pots for gain control. I added vintage on/off switches, new rca’s, belton sockets, 5 way binding posts, and new power cords. I repainted the transformers but left the original patina on the chassis. Left the can cap in place just for looks. The second unit I refurbished had a bad 100ohm balance pot, so I went back thru and changed them both so that they matched. I haven’t fired the 2nd unit up yet. I’m always scared after a complete tear down and rebuild. I will look over it a couple more times and then give it a try. 45152F64-DA73-4534-A90A-536A1EBA0716.png1359F88A-297D-415E-8B42-6C7F4493EF6A.png9E9C6657-E25F-48D4-A021-001CFF66339C.jpeg5E44CAB5-1325-4B43-8B6A-B16EC42858ED.jpeg
 
So I hooked them up and all is well. One is noisy compared to the other. Sounds like ground hum to me. I will go back through the noisy one and check solder joints again, especially at ground. Is that the right course? They are built almost identical to one another, using the same type and color wire, and the wires even run along the same paths in each unit. I only had time to try them on some crappy speakers. I will know more after hooking them into the main rig.
 
60hz hum is usually ground or something with the heater circuit. 120hz would be main power supply filter problems.

Only thing that jumps out as a maybe, check the insulator bushings under your input jacks. I can't see them clearly to see if one on the inside of the chassis might be missing. Also looks like maybe the line cord ground goes to a different spot, but not 100% on that either.

just a comment because I can't help myself, it looks like the fuse is on the neutral side and the switch is on the hot. Personally I'd rather see both on the hot side, with the fuse being first.
 
Thanks gadget. I will fix that. That makes sense. I think I will use the little tone generator app I have on my phone and send a 120hz signal thru the quiet amp just to see what that sounds like compared to the 60hz. I think the hum I am experiencing could very well be 120hz.
 
Also, try swapping the input tubes between the amps.

A heater-cathode leak in a small-signal tube, can definitely cause hum. Swapping them out will let you know if it's a tube problem, or if it's somewhere else in the amp.

Regards,
Gordon.
 
I’m blessed by audiokarma royalty in this thread! At least I feel like I am! Thanks for all of the input guys. I haven’t had time to do much of anything since initially firing them up on the crappy set of speakers. I would be really shocked if I forgot to solder anything at all inside the amp. I’m a check it 20 times before I fire it up kinda guy. Maybe a cold solder joint though. After running a tone through those crappy speakers it def seems like a 60hz hum that I’m experiencing. I will swap tubes just to rule out the obvious and then dig around while blasting things with a soldering iron to make sure everything is solid. I can’t remember if I floated the rca jacks or if I let the ground portion touch the chassis in order to be grounded? I feel like I didn’t float the rca jacks? That’s the way the original amps were built, and the way my friends were built as well? If I did float them then that would be my problem cuz they don’t run to ground. That’s what makes me think they are touching the chassis. Should I float them and then run them to one of my other ground points? Again, they weren’t originally built that way.
 
Imagine my surprise cruising the new posts and I see a thread with one of my amps in it (the reference amp). That's some clean work Jon! Did you solve the 6L6 vs EL34 mystery? Which way was the socket wired originally? Of course it doesn't much matter since you completely rebuilt them but that was news to me on these amps. Of course CoastinHome is the West-Coast expert on all things Ampex so it doesn't surprise me at all that he chimed in. There must be a bell that rings every time an AK'er mentions the word Ampex, just like Tom Bavis always shows up when a schematic is searched for.
 
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After getting to know the ampex brand and the amps, searching the transformer numbers, these are definitely the el34 amps. The 6L6 amps came with smaller transformers with different serial numbers on them. You can even see in my chassis where there are screw holes present where a smaller transformer could be installed. I think this was there because they built the 6L6 amp into the same chassis as this one. The person I bought these from just had 6L6 tubes installed in the amp when they shouldn’t have been. I think these amps were the earliest versions of the el34’s that ampex made because the transformers numbers match up, the size matches up, but they painted them black and they didn’t have the Triad sticker, like the later version that was grey, and had the sticker present. I will search the serial numbers and clarify this statement so others will know.
 
Ampex built the el34 and 6L6 based amps into the exact same chassis. They claimed 40 watts on the schematic of the el34 and 25 watts for the 6L6. The output transformer is noticeably larger on the el34 version.
byoungman1 has a post here on audiokarma from 2013 where he gets his 6L6’s going, but also includes a pic of both sets of amps side by side. The output transformer in the 6L6 was serial number 58-009, while the el34’s output transformer was 658-0027. Here’s his post
https://audiokarma.org/forums/index.php?8CD81B1B-8C89-4E55-82F1-8744675B36B1.png threads/1960-ampex-6l6-mono-amps-finally.509764/page-2
 
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