Matt J wrote: ↑Tue Jul 03, 2018 7:55 pm
I rewired the R/J switch to match the original #183 layout. I noticed on it that the lead from the bass cap to the switch attaches at the same solder point as the wire going to the bass pot. That's how I have wired mine.
The volume drop is no longer there, but it still doesn't seem that the PAB function is working. There's no change in volume or tone at all. I might have erred in using an On-Off-On switch in the first place and may just have to pull the whole thing out and replace it with an On-On switch.
Also, the FET now seems to be backwards. There's a volume drop when it is engaged. It's working, you can tell since the FET gain pot works, but switching it on just cuts out at least a third of your volume.
- Matt J.
Matt,
I biased my FET board with Martins 18v battery method to get roughly 9v from the 18v. After that I found if the FET pot is around 12 o clock you will get a nice little bump of clean boost when engaged.
I would say unity gain would be around the 10 o clock mark.
Here's a link to a picture of the R/J switch and PAB relay wiring: https://tubeamparchive.com/viewtopic.ph ... 37#p145137 The bottom two lugs are jumped, and from there a yellow wire goes to right, to the bass cap, and a white wire goes to the left to the bass pot.
I'd try biasing the FET again. If that doesn't work, it's possible it has been damaged by heat or the reverse polarity incident.
martin manning wrote: ↑Tue Jul 03, 2018 9:51 pm
Here's a link to a picture of the R/J switch and PAB relay wiring: https://tubeamparchive.com/viewtopic.ph ... 37#p145137 The bottom two lugs are jumped, and from there a yellow wire goes to right, to the bass cap, and a white wire goes to the left to the bass pot.
I'd try biasing the FET again. If that doesn't work, it's possible it has been damaged by heat or the reverse polarity incident.
Got the PAB working! Apparently the issue was the On-Off-On switch I used. I installed an On-On replacement and wired it as it originally was. PAB now has a nice subtle kick to it. It doesn't really knock you in your face, just adds the right amount of boost to your tone. That's how I like it. I've played some Dumble style amps before and found the PAB seemed to cause too much of a volume jump when engaged.
As for the FET, it's still causing the volume drop. I replaced the NTE452 with a new one after the polarity accident. There could still be an issue from when I tried to bias it. I'm probably going to pull it out and redo the input as "normal only" for the time being.
I'll work on retrying the bias method and redoing the board later. Right now I want to focus on tweeking the clean and OD channels, especially the internal OD and PI trimmers.
I've had a ton of fun after work this evening giving the amp a go! I'll post more pics later.
One thing I'm looking forward to trying with this amp is a good delay pedal.
Have any of you who have built this circuit, or something similar to it, have any recommendations for a decent delay pedal I should check out? I'm hoping to pair this with a Dumblator one day, but for now I'm just looking for a basic delay unit to try it with.
Matt J wrote: ↑Thu Jul 05, 2018 7:00 pm
One thing I'm looking forward to trying with this amp is a good delay pedal.
Have any of you who have built this circuit, or something similar to it, have any recommendations for a decent delay pedal I should check out? I'm hoping to pair this with a Dumblator one day, but for now I'm just looking for a basic delay unit to try it with.
- Matt J.
I use/have used in the loop. TC flashback, TC alter ego x4, TC nova delay, Eventide H9 , Line 6 M5 delays. I think the Strymon El Cap changed the sound/Tone too much in the loop, I think it's better if you use it in front of an amp, old school style.
martin manning wrote: ↑Thu Jul 05, 2018 10:49 pm
Be mindful of the voltage potential at the send jack- it could damage the input stage of your FX if you bring the master up to high.
Thanks for that tip!! I'll test things next time I fire up the amp!
- Matt J.
I ended up removing the FET circuitry completely and just leaving it with the stock input and PAB/OD footswitching options. I'm happy enough just proceeding from there. I've found the amp to have PLENTY of gain on hand without the FET. The cleans sound really good as is, so I'm content to leave thing as they are for that part of the circuit.
I did try the LNFB on V1B. It was interesting, especially in the cleans, but it left the response a little too flat in the OD mode. If I was voicing this amp to have a little less gain I would have left it in, but since I want this amp to be on the higher-gain end of the ODS spectrum I removed it and returned the circuit to stock.
I'm still playing with the OD trimmer and drive controls to get that "Sweet Spot" that works with the JJ Gold Pin 12AX7 that's currently in V2.
One thing I found that affected the tone the most was the Bright switch. I used an On-Off-On switch with 100pf and 220pf ceramic caps. Engaging either of these added lots of gain and sizzle to the tone, so much so it sounded nearly like an overdriven Marshall. The negative was that it added far too much hum to the signal. Adjusting the preamp volume does not affect this hum that much, but it becomes unbearable after turning the Master past 4. Both of those caps created too much gain and noise to me, so I'm going to replace that with a lower value and different type of cap. Any opinions on how silver mica or another form of capacitor would work in that position?
One other thought I have had, and would love to hear feedback on, is eventually replacing the four Xicon carbon film resistors in the OD section with some NOS Pihers. A 68k, a 100k and two 180ks. I know the layout for the original #183 has a 68K Piher as the grid resistor on V2A, but I have never used Pihers so I can't attest to their noise level or effects on tone.
Any suggestions on this? Could swapping out the Xicons in this section have that much impact on the OD tone?
Played around with the bright switch some more today. I ended up going with a single 220pf, it had the best balance and didn't seen to inject as much noise in the OD channel in the previous ones. I also replaced the carbon film 68K grid stopper on V2A with a metal film one just to see how it sounds.
I've worked some more on the OD trimmer and think I have it set just about where I want it. I'm going to give things some good burn in time before playing with it anymore. There's also now around a ~7 VDC difference between the plates of the PI.
There's still a lot of hum that comes up when turning up the master volume. I know since this design has more gain in the circuitry than a traditional Dumble circuit, but I'm still curious if it could be a component, tube, or something with the wiring.
I'll post some voltages tomorrow, that afternoon I'm going to load everything into the speaker cab.
Here's some updated pics. If anyone spots anything I may want to consider changing.
- Matt J.
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I checked the voltage on the send jack with the master up, came out to about ~3.3VAC. Seems small, but would this amount be cause for concern with some pedals?
- Matt J.
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I'm quite proud of how it has turned out so far! After ~8 years of building amps I think I've learned more about audio circuits working on this thing than with anything else I've built! It took me over half a year to gather together and test all of the parts I've been wanting to use for it. I can't wait to get everything finalized on it and to start working on a Dumblelator for it!
There's a volume drop when it is engaged. It's working, you can tell since the FET gain pot works, but switching it on just cuts out at least a third of your volume.
