5 watt SE guitar amp head with sidetone buzzing problem when strings are strummed

[rock soderstrom]

Well, as I said, the original problem is solved so far.

Just for the sake of completeness, a few comments from my side, as unlike others I just had the amp in front of me.

but a 4 watt 220R resistor on the B+ between the reservoir cap and Node A is both pointless and an unnecessary source of heat that could harm the PCB, especially if it's mounted directly on the board.

This resistor is not pointless and I strongly advise (for future readers) not to simply remove it. B+ (the heater voltage too) are at the upper limit in this amp.

The additional filtering does no harm and the resistor is not mounted directly on the PCB but approx. 2 cm above it. It is sufficiently dimensioned and therefore does not cause any heat damage to the PCB.

Basically, PSUs can be designed in different ways, the additional B+ filtering has nothing to do with the original (now solved!!) problem.

If I've ruffled some feathers, I apologize. That wasn't my intention.

There's not much left of the points you mentioned in your first post in a rather arrogant manner? Apology accepted.

You seem to like telling your tech anecdotes, late at night with a glass of wine perhaps? You've come to the right place, so once again a warm welcome.

 

[rock soderstrom]

Can you confirm something to me? This is a Harley Benton GA5, correct? Can you tell me if the power transformer bell ends are facing left to right or are they facing front to back?

Yes, Harley Benton GA5H, the head amp version of the GA5.

 

[rock soderstrom]

Well, one of your first posts on this whole forum is telling off the "for funsies" project of a(n arguably) long-time and relatively respected member here.

yes relatively respected, about 27.89% of the members respect me here, but I have more followers than you!

 

[Khron]

but I have more followers than you!

Good, i'm not a fan of stalkers...

(... unless we're talking video games, but that's a different story altogether... )

 

[rock soderstrom]

Good, i'm not a fan of stalkers...

Followers are sometimes really not that easy...this gentleman can tell you a thing or two about it.

 

[ruffrecords]

@rock I am confused. Can you remind me exactly what it was you did that fixed this problem?

Cheers

Ian

 

[Alva Goldbook]

This resistor is not pointless and I strongly advise (for future readers) not to simply remove it. B+ (the heater voltage too) are at the upper limit in this amp.

Heya, would you be able to tell me the rectified B+ voltage?

Thank you for posting the picture of the amp chassis. The power transformer is indeed mounted incorrectly. The magnetic field would indeed go from left to right, and effect the sensitive preamp section of the amp.

Given the rather amateurish power supply design, plus the incorrect orientation, I'd worry if the designer of this amp knew to put in a correctly sized power transformer. Power transformers should be suited for their current demands. Too low, and the voltage will spike too high, stressing the power transformer. Too much current demand, and the voltage will sag low, which will also stress the power transformer.

EL84s are pretty robust. God knows Vox abuse the hell out of them. While the technical limiting plate voltage is 300v, most common designs put them around 330v. I just sold a 20w Marshall clone that had the voltage closer to 360v on the EL84s. This is fine. What's important is dissipation, and keeping the wattage at around 70% in a Class AB amp or up to 90% in a Class A amp like this.

When you work as an amp builder it's easy to over look some aspects of design. When you're a repair tech you see blunders everywhere. It's like playing a game of Where's Waldo, except Waldo's name is "That Could Catch Fire". It is always poor practice to place any high current low ohmage resistor in a tube amp power supply. You avoid this by using the properly sized power transformer.

Furthermore, no B+ circuit is perfectly silent. There is always some voltage ripple. This is why in any properly designed supply, you do the proper RC filter calculations to make sure any noise is below 20Hz so it's not audible. A supply that has a 220 ohm resistor and 22uF filter caps will have a frequency cut off of 32.88Hz. No competent amp designer would have ever done that. This could have very well been what was causing your oscillation issues.

Please tell me the rectified B+ voltage.

 

[rock soderstrom]

@rock I am confused. Can you remind me exactly what it was you did that fixed this problem?

Cheers

Ian

Moving the input and output of the V1b gain stage further apart on the PCB, re-routing of two tracks, grid stoppers and rearranging the wiring harness solved the problem.

 

[rock soderstrom]

Heya, would you be able to tell me the rectified B+ voltage?

The rectified voltage in the original state was 351V. Please send the remaining complaints to the original designer. I don't want to defend the work of a guy I don't know here. The amp had bigger problems than the alignment of the transformers.

Edit: original schematic added in post #2