Bogen chb-35a conversion. Strange feedback issue

[thehoj]

I recently decided to rebuild a bogen chb-35a into more of a guitar friendly amp.

I took it from this:
http://www.schematicheaven.com/hifia...gen_chb35a.pdf

To this (at the moment):
http://vwtweaked.ca/images/jogen%20c...s%20bright.jpg

Pretty much a complete redesign of lots of it, all new caps and resistors.
Initially I based my conversion off of the ampeg sb12 portiflex, but then it mutated into I don't even know what.

I have the power tubes biased at about 70-80% of max plate dissipation and
I'm very happy with what I've got now for tone, tone control, and overall sound. I may play around more with the tone stack, and bits of the preamp section, but I think it'll basically stay as it is.

I just have a strange issue now with controlled feedback.. Not sure if that's the right terminology for it, but what I mean is that if I crank it right up so that it's being overdriven nicely, and then I bring my guitar close to the speaker to let a note I'm holding feedback through the speaker, rather than getting a nice controlled feedback (like I do on my traynor ygm2) I get a weird squealy oscillating feedback..

Is there a section of the amp that is a likely culprit for this?
Any thoughts on the strange feedback?

[Chuck H]

My first thought is that your new amps frequency curve and power is just more apt to cause your pickups to go microphonic. Less likely is the possibility that the higher frequency feedback note is somehow causing the amp to oscillate. And even less likely (but still possible) is that the amps internal layout and/or speaker is emitting enough EMF that when you move your guitar near it begins to oscillate.

Do the gain settings and power seem similar on your Traynor and your Bogen? Is your Bogen brighter than your Traynor? Have you tried another guitar? Have you tried seperating the amp and speaker to see if it may be proximity to one or the other causing the problem? Does it happen with single coil and humbucking pickups?

Chuck

[thehoj]

If the first scenario is the case, does this mean that adjustments to anything that will affect the tone of the amp could improve or worsen the issue?
ie) tone stack components, coupling caps, resistors, etc..?

The gain settings and power are sort of similar between the traynor and this bogen, but they are very different amps, I was just using the traynor in an example of the type of controlled feedback I thought sounded good.

The Bogen is a pretty bright amp, I have some caps over the volume control to increase the brightness.

The amp and speaker are not close to each other at all. The amp is a standalone unit on my workbench at the moment, and the speaker is in a cab several feet away

I only have one guitar.. with humbuckers, so I can't test it with single coils.

The squealy oscillating feedback is always exactly the same though, and I noticed that it didn't matter if I cranked the volume on the amp way up to achieve overdrive, or keep the amp clean and use my boss ds-1 for overdrive, the feedback was identical.

My first thought is that your new amps frequency curve and power is just more apt to cause your pickups to go microphonic. Less likely is the possibility that the higher frequency feedback note is somehow causing the amp to oscillate. And even less likely (but still possible) is that the amps internal layout and/or speaker is emitting enough EMF that when you move your guitar near it begins to oscillate.

Do the gain settings and power seem similar on your Traynor and your Bogen? Is your Bogen brighter than your Traynor? Have you tried another guitar? Have you tried seperating the amp and speaker to see if it may be proximity to one or the other causing the problem? Does it happen with single coil and humbucking pickups?
Chuck

[Chuck H]

If the first scenario is the case, does this mean that adjustments to anything that will affect the tone of the amp could improve or worsen the issue? ie) tone stack components, coupling caps, resistors, etc..?

Yes, but this would change the tone and high end responsiveness of the amp. A better plan, if this is the case, would be to replace your guitars pickups with less microphonic units.

The amp and speaker are not close to each other at all. The amp is a standalone unit on my workbench at the moment, and the speaker is in a cab several feet away

This would indicate that the problem is not a microphonic issue with the amp itself. More likely the guitar. The Bogen's frequency curve just happens to set it off. Changing the tonestack or other variables in the preamp is not likely to help much because when the amp is overdriven most of it's frequency response is controlled by the power tubes, output transformer and speaker. All of which are on the back side of the amp's preamp circuitry.

I only have one guitar.. with humbuckers, so I can't test it with single coils.

This would eliminate the possibility of there being any excessive EMF from the amp or speaker coil because a humbucker would cancell most of that before it ever reached the input of the amp.

The squealy oscillating feedback is always exactly the same though, and I noticed that it didn't matter if I cranked the volume on the amp way up to achieve overdrive, or keep the amp clean and use my boss ds-1 for overdrive, the feedback was identical.

This would indicate that whatever is creating the oscillation is remaining constant. Since you greatly vary the conditions on the amplification end, by changing between a DS-1 and the amps natural overdrive, I would speculate that the guitar pickups are microphonic.

Most guitar pickups are microphonic at some frequency or sheer volume level. I HATE pickup microphony so I use Seymour Duncan pickups most of the time. They have a wide range of vintage and hot rod styles and are NEVER microphonic. The best "potting" of any pickup IMHO (I've had trouble with other major brands). I would try another guitar, a friends or take the amp to your local music store to try it with other cabinets and guitars. If the problem persists at the same frequency the it may still be the amp. If the problem goes away or there is a frequency change then it's the guitars, and how they react with the amp. Maybe it;s too bright, and maybe your pickups are too sensitive.

Chuck

[thehoj]

I would speculate that the guitar pickups are microphonic.

Is this a common thing? The guitar is a Gibson LP studio with factory pickups.

[loudthud]

I just have a strange issue now with controlled feedback.. Not sure if that's the right terminology for it, but what I mean is that if I crank it right up so that it's being overdriven nicely, and then I bring my guitar close to the speaker to let a note I'm holding feedback through the speaker, rather than getting a nice controlled feedback (like I do on my traynor ygm2) I get a weird squealy oscillating feedback..

Acoustic feedback will change frequency with slight changes in guitar position. Electromagnetic feedback will only change the amplitude with slight changes in the guitar's position.

[olddawg]

Is this a common thing? The guitar is a Gibson LP studio with factory pickups.

Pickup squeel is very common at higher gain settings in close proximity to an amp. If you use a 15' cable and get away from the speaker it should go away or at lest the tenancy reduced if you turn one way or another. If it doesn't, there is a problem with the amp. LP Studios are not particularly known for microphonic pickup problems. Have you tried swopping V1 with another tube? Preamp tubes can become squeely and microphonic too. Have you done a pencil test?

[thehoj]

Pickup squeel is very common at higher gain settings in close proximity to an amp. If you use a 15' cable and get away from the speaker it should go away or at lest the tenancy reduced if you turn one way or another. If it doesn't, there is a problem with the amp. LP Studios are not particularly known for microphonic pickup problems. Have you tried swopping V1 with another tube? Preamp tubes can become squeely and microphonic too. Have you done a pencil test?

I don't get uncontrollable feedback at all, it's just when I say bring my guitar fairly close to the speaker purposely to generate some feedback like this (bad example) [ame="http://www.youtube.com/watch?v=6C_zO_zguUQ&feature=fvw"]YouTube - How to generate and use Feedback with your Guitar and Amp[/ame]

Rather than sounding like this (basically), it starts to oscillate squeal.
I haven't tried a different tube in v1. I'll try that.