Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Garrett335 on August 25, 2021, 10:23:32 pm
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Hey guys!
I wanted to see if anyone knows what I'm doing wrong here.
I have successfully built the 2022 18w amp many times, and I wanted to add a TMB pre to it to give a little more versatility.
I changed the entire pre to match the Stout input/pre up until the PI grid of v2a.
V4 starts red plating! The plate and screen voltages look normal on the output tubes.
I did hook up the scope to it and the signal coming out of v4 is way larger than v3, something is imbalanced.
Any thoughts? Much appreciation
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Here's a photo of the gutz btw :)
(It's rough, it's the Frankenstein chassis for testing stuff out...)
(https://i.ibb.co/ykz52Lq/Screen-Shot-2021-08-25-at-8-21-01-PM.png) (https://ibb.co/pKHZqCd)
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I did hook up the scope to it and the signal coming out of v4 is way larger than v3, something is imbalanced.
Flip flop V4 tube with V3 tube.
Does the red plating follow the tube or stay in the same V4 socket?
If it follows the tube, its the tube.
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Screen grid stoppers as per the Stout would be a very good idea.
It may be that the change to a cascading gain preamp is bringing out layout / lead dress issues. eg is that the output jack between V1 and V2?
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@willabe - It stays with the V4 socket : /
@pdf64 - I'll add those and see what happens 1M 3W looks like. And yes that's the output jack. It previously had 3 jacks but I removed two of them.
What's strange is that this amp was working fine as the 2022 build. All I changed was the preamp, all of the power section and output section stayed the same.
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I'll add those and see what happens 1M 3W looks like.
1M screen grid stopper? 100ohms to 1k is more usual. 3W seems overkill for EL84.
And yes that's the output jack. It previously had 3 jacks but I removed two of them.
What's strange is that this amp was working fine as the 2022 build. All I changed was the preamp, all of the power section and output section stayed the same.
The change cascaded the preamp gain stages.
That’s a significant thing.
An output circuit in close proximity to an input circuitry is a bad idea.
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@pdf64 oops! I meant 1k 3w based on the stout schem. I don't mind putting a smaller W resistor ha.
I've never had a problem with the Output jacks located there. I can try moving it, but would that cause redplating?
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I’m thinking that the issue may be due to oscillation.
The absence of screen grid stoppers, the issue occurring following the system gain being increased, and the siting of the output jack in close proximity to the input stage are possible contributory factors to that.
On a safety note, the back terminal of the fuse holder should be connected to the hot feed, the side contact should be wired to the fused output.
That reduces the risk to the user if they’re a little careless when messing with the fuse.
And the chassis safety ground lug is connected to the chassis using the IEC inlet fastener. Those can’t take much torque before the plastic of the inlet body splits. It’s better to use a dedicated fastener that can really be tightened up, so that the star washer bites hard into the chassis metal.
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Thank you @pdf64
I'll make those safety changes, as well as add the screen grid stoppers/output jack relocation this morning!
Wish me luck ha.
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@PDF64 !!!! Thank you!!! Thank you!
The screen grid stoppers seemed to help it, no more issues, and it sounds awesome!
Going to watch it some more to see what happens but so far so good. I appreciate you taking the time to help me out!