Repairing an original JMI Vox AC10 SRT(!!!!)

[@steveennever9905]

What an utterly beautiful sounding amp.

[@Strumbum01]

Great sounding amp, congrats! I’d love to build myself a clone.

[@YeatzeeGuitar]

It's a great one to clone

[@Strumbum01]

@@YeatzeeGuitar I appreciate that schematic. I may pursue this but not sure about that reverb mounted in the chassis. Is it noisy? Thanks again!

[@YeatzeeGuitar]

@@Strumbum01 definitely can't clone the reverb aspect of this amp as is, it uses a weird setup different from Fender or the like

[@cgavin1]

Bloody canned caps! Always an "arts and crafts" project to replace. Great job as always!

[@YeatzeeGuitar]

That's a great way to put it lol

[@stevendehaan1078]

Congrats on the new amp! Great work, thanks for the detailed circuit review. Sounds awesome.

[@YeatzeeGuitar]

Thank you! I'm so excited!

[@dyamariv3628]

Yes Sir, sounds great Brother! Loving this series, you've had some killer amps recently! PS. I'm still salivating to hear the JMP 100 in action at your gig, hope it wen't well!

[@YeatzeeGuitar]

Thanks! And unfortunately my gig got canceled! The tour bus for the artist broke down so they never made it to the venue, and we were the opener. Bummer! I've got another gig at a big enough venue to play it at on the 4th, so that's the aim now. More to come!

[@dyamariv3628]

@@YeatzeeGuitar Bummer on the cancelled gig but great to hear you have another oportunity for that bad boy to rock. Looking forward to it!

[@YeatzeeGuitar]

@@dyamariv3628 As am I! I've played with 4x12's live a few times, but never with a 100w Marshall on top! I'm sure the fryette Power Station will be getting some service

[@jimmello7363]

Awesome job yet again .. that sounds good ! Hope you film the gig using this amp ? Crystal clear tones and it sounds powerful. I'm totally impressed n the sound cause I have never heard of this amp til now but they are extremely hard to find the ones found don't work and missing parts and there asking $grand for it yikes! But I did find a few for sale asking around $3500-$7000 , but the UK has a few of them 1964 for 4 grand . So I have learned a whole lot from you since I been hooked on your channel,thanks for the schooling ! Let us know about the gig !

[@YeatzeeGuitar]

@@jimmello7363 shoot me the link, i want to see those for sale! I couldn't find any. Thanks BTW!

[@jampay44]

Cool video, I like that Pittsburgh tray!

[@YeatzeeGuitar]

Very handy

[@jutukka]

If screen voltages are a bit low, it is very normal that plate current is low too. On the other hand, a bit low screen voltages is a good thing because it protects screens from overheating when output stage is driven to clipping. In many amps screen voltages are a bit too high compared to plate voltages and especially if screen resistor values are too small as well, tube life can be quite short if amp is played cranked up all the time.

[@paulshurmon138]

Thanks for the video.

[@benfischer6601]

If you need more plate voltage I would check the circuit around the 12AU7. Since those are biased anywhere from 5-30mA, even a slight variation in the cathode resistance of those preamp stages can create a major difference in the B+, since it's all going thru the first dropper resistor. You could even bias the 12AU7 a little cooler by changing a cathode resistor with probably zero ill-effects

[@YeatzeeGuitar]

The current plan is to get to know the amp a bit as-is, since it's very very close to the original spec/schematic now. All resistors test great, those that didn't / were old have been replaced at this point so she should be good to go for the most part. We're still getting low plate voltage, but the tubes also tested fine (mentioned in another comment, but I got a new 12AU7 and a NOS 12AU7 to test out and see if that made a difference. I also tried 12AX7's instead). I haven't calculated current draw yet for the preamps but it's on the list of to-do's. Another option on the board would be just reducing the dropping resistor from 1k to 820 or something.

[@benfischer6601]

@@YeatzeeGuitarword! Yeah it sounds like it’s working well and is stable, which is the most important thing. And yes, replacing that power resistor would definitely increase plate voltage as well. Any old coupling caps can also be prone to leakage which can lead to more current draw on any gain stage. But you may have already gotten the amp to where it is “meant” to be, within the tolerances of manufacturing

[@jefffree6990]

Love it!

[@MichaelSmith-rn1qw]

Regarding my comment on your first video on this amp, the tube you "may" want to try in V2 is a 12DW7/ECC832. Section 1 of the tube (pins 6, 7, and 8) is the high gain 12AX7/ECC83 spec (gain factor 100) and Section 2 (pins 1, 2, and 3) is the low gain 12AU7/ECC82 spec (gain factor 17). This will give you more gain while still using the 12AU7 triode as the driver. Another candidate is the ECC823, which is the same as the ECC832, except the two triodes are reversed. I don't remember how your amp was wired, but I think either tube would work if you wire the pins correctly. Either of these tubes cost about the same as a regular 12AX7.

[@YeatzeeGuitar]

Don't have any of those weirder tubes around, but I have tried a 12AT7 and 12AX7 in V1-3 and all worked well. More on that in the future!

[@MichaelSmith-rn1qw]

@@YeatzeeGuitar I've not used any of those weird mixed triode tubes myself, but noticed AES has them in stock when I was buying some tubes when they had their 15% off sale a week or so ago. JJ currently makes them so obviously there is a demand for them.

[@James-Customs]

Love the two videos, great work. I was confused why your plate voltage was so low compared to the B+. But just realized the linked schematic has an error? Is the OT feed after the 1k with the screens? The Schematic shows 390v to the ot but says 295v on the plate, which wouldn't make sense and had me confused. I Did find other schematic showing ot and screens on the same node, which would match your numbers. So yeah if plate and screens are feed off the 1k dropping resistor, would a 750ohm put you in the 300v ball park.

[@YeatzeeGuitar]

Yep it's got that one error, I can't remember if I mentioned that in any of the footage shot do far but yes definitely noticed that mistake. I also confirmed on photos of other original examples that they're the same as mine regarding the location of the OT center tap off the 2nd cap post dropping resistor

[@oldasrocks9121]

I agree with benfischer, 12AU7s can mess up your voltages and current draw across the entire circuit, they are total current hogs. I'd start by temping in a new or nos known as good 12AU7, measure your plate voltages again.

[@YeatzeeGuitar]

I bought a new JJ ECC82 as well as a NOS 12au7 to test that theory, and there was no change iirc. I didn't film it, but I plan on doing a deep dive into tubes with this amp in the next part of the series. Doug and Pat for example are "famous" for going 12ax7 in V1 & V2, and then I believe a 12AT7 in V3 with the 12AU7 remaining in the PI. I did also measure with that tube compliment in it and there was not a real noticeable change. The amp came with 12AU7's in V2-4, 12ax7 in V1.

[@oldasrocks9121]

@@YeatzeeGuitar Tube rolling or redoing the circuit so all those new tubes hit their center points?

[@David-gx9lh]

Looks like Champ Electronics has ceased trading.

[@YeatzeeGuitar]

That would explain why they never replied to my email lol

[@Dave-vq9yo]

sad times, it looks like John Chambers own website after Champ Electronics has also stopped trading, his website went down earlier this year and never came back up

[@huddyskiba1]

10:46 🍻🫡

[@drpaul0]

Did someone add balance resistors to the heaters not realising it was centre tapped? Don't think it's a problem as the coils have lower impedance then the resistors, I guess one way is lifting the transformer centre tap if it buzzes like crazy they weren't balance resistors 😊

[@YeatzeeGuitar]

I can't remember if I kept the portion of me talking about that in the video or not, but yes super weird. There are two wires coming off the PT going to ground. Schematic shows just the high voltage center tap so I'm not really sure what's going on. Left it for now, but definitely needs some further investigating.