Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on November 18, 2022, 11:58:18 pm
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Hi guys, Got some 5F1 boards made, so.....
Not built with the usual components.
(https://i.imgur.com/vFX3M8J.jpg?1)
(https://i.imgur.com/pjB5QtK.jpg?1)
(https://i.imgur.com/jZzjN3d.jpg?1)
(https://i.imgur.com/3ryEhEv.jpg?1)
(https://i.imgur.com/pmcXzwa.jpg)
(https://i.imgur.com/Mb1rwZH.jpg)
(https://i.imgur.com/OyBcoOA.jpg)
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Ciao TIMBO
Nice cabinet 👍
About your unusual PS .....
Back to Back Transformers .... OK, but, sorry to say, ..... if you feed a 15v input transformer with 9.5v all Is fine, BUT to feed a 9.5v input transformer with 15v Is not a good practice, may be all Is fine BUT easily the core of the second transformer will go to saturate and become hot
Your chanche for an aceptable result is the use of overrated transformers
Franco
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Hey Franco,
Both transformers are rate at 1A
The first (input) transformer barely gets warm.
I'll keep an eye on it.
Thanks
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Are the HT (A-C) supply voltages design or actuals?
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Actual
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Thanks.
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Hi guys, The "Champ" is all but done...
(https://i.imgur.com/9z8CGRG.jpg)
(https://i.imgur.com/OqBkPBy.jpg)
(https://i.imgur.com/YVY5tqQ.jpg)
(https://i.imgur.com/ks56YhA.jpg)
I'm sure I've got the squealing thing sorted by removing the bypass cap on V1a
I now have another problem.
It sounds like DC on the volume pot.
Easy, replaced coupling cap previous to the pot, no change
Changed the pot just in case, all new pots
So I'm kinda out of ideas ATM
The scratchy sound is only on a small portion of the rotation between 7 and 9 on the dial.
:dontknow:
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is that the sweet spot for the amp, 7-9?
might try and make it "fixed" with a 1 M resistor just to insure it's not something else nippin at you.
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... It sounds like DC on the volume pot. ...
Did you measure to see if there was DC on the pot?
I ask because I wonder about the possibility of grid-current in the 12AX7.
- A DC Volts measurement while rotating the pot may help diagnose.
- If you do get DC volts, try removing the tube & repeating the test.
- If you still get DC volts with the tube gone, it could be a board defect
(I'm not saying the probability is high of a pc board defect, but I once worked in a pc board manufacturing house & saw defects between traces inside layered boards that were very difficult to troubleshoot)
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NICE!
How do you like the WGS speaker?
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Hey guys.
I'm getting a fluctuations of 3mV with valve in and slightly higher with valve out
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I don't believe 3mV DC is considered "leaking"
the "only between 7-9" has me head-scratching, especially with a new pot n cap.
what happens if you disconnect the NFB?
what happens if you remove "X" from the PA cathode and move it to ground? (I'm assuming this is elevated heats?)
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... I'm getting a fluctuations of 3mV with valve in and slightly higher with valve out
Your meter probably has a 10MΩ input impedance. 3mV / 10MΩ = 300 picoamperes.
That is probably environmental and/or unavoidable. Doesn't seem to be the issue.
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Try squirting some deox/CRC into the pot guts
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Changed out the pot with another new, no change
Artificial heater CT disconnected from output valve, no change
NFB disconnected.
Valves changed
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sell it to a kid from NZ :icon_biggrin:
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Hey guys, Really got no idea why the scratchy pot.
I replaced the preamp socket and again no change.
I have used the back to back transformers in other builds with no problems.
So other than gutting it and rebuilding it on another board or point to point, I feel I have no other option.
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did you temp-in a fixed resistor in place of the pot? use the guitar volume to "see" if there's still "scratching"
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Hey shooter
I disconnected the first .022 from the pot and connected it to the wiper of the pot so I have the amp at full volume.
The ground and the wiper are still connected to the board.
Turning the volume there is still a scratchy noise
Doing your last request had no scratchiness from the guitar volume
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I believe you have a pcb connection flaky, trace, pad, wire....
"fly" the coupler and a fixed resistor to the PA socket, bypassing the pot, pcb, and wires.
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Hey guys, I really had no other choice but to build another board.
(https://i.imgur.com/tXlnWiJ.jpg?1)
NOW it sounds like a 5F1.
As you can see there is a spare eyelet...
I have a 68uf tacked in the cathode, this was part of the problem with oscillation, now there is no problem with oscillation with a bypass cap.
Sounds great with or without, so possibly a switch on the back of the amp.