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**Rob Mercure** · 10-16-2006, 02:08 PM

BD,

Well, sounds like you've mostly diagnosed the problem - the oscillation - now to find it. Did the amp oscillate before you rewired the V1 socket? (unnecessary in my opinion - the 6EU7 is the same tube with a 6.3 V heater and seems to last forever in these amps) And it's only when you use one side of the tube (one input?)? Since you converted to the AX7 I assume you've tried a different tube? If so I'd go over the wiring carefully. Oscillation is positive feedback - I assume you know that - but one stage of tube amplification (except cathode followers, etc.) inverts the signal so plate to grid feedback is negative - no oscillation. What you've got is feedback over at least two stages so look for any sort of coupling from the plate of V2 to the grid of the 1/2 of V1 causing the problem. Did you reroute wires (probablly) when you removed unwanted features? Restore the amp to as close to stock as possible - without rewiring V1 socket - and see what happens.
Since you don't have the same problem with the second input I wouldn't suspect the filter replacements but you might just want to go over them "for the hell of it" and perhaps parallel a smaller signal type cap - 0.01 uf or so at 500 VDC - across your last filter.
The schematic doesn't list - as best I can tell - the voltage on the 6EU7s/12AX7s but Bogen typically ran these quite low along with the grid leak bias - about 45 VDC if my memory serves. You haven't swapped the plate resistors for more gain have you? These stages work pretty well for guitar input if left mostly stock but they really don't "tweak" well and if you want to change them it's often easier to just up the supply voltage and put in a stock cathode bias stage than to mod the grid leak.
Hope this helps.

Rob

**Blindog** · 10-17-2006, 01:40 AM

Rob,

Thanks for the reply. I just came up from the "Laboratory" after playing around with the input wiring routing and that is the root of my problem. The inputs came from the rear originally, and I had input one going to the front of the amp and input 2 not connected to the chassis yet, still towards the rear. When I removed the input jack from the front and moved it towards the rear the squeal stopped. So it looks like I need to find a clean route to front. I changed the V1 to a 12AX7 on a suggestion from someone in this forum who had modded a lot of Bogen iron. (Don't remember who) I have not changed any plate resistors, don't think I will either. I am stuck on what to do with the other input. Could I cascade the 2 stages together safely? Again, thanks for your input!

Mark

**Enzo** · 10-17-2006, 03:44 AM

Or use shielded cable from the board to the jack

**Blindog** · 10-17-2006, 05:26 AM

Well, the cable is shielded, but I will make sure it's properly grounded. Thanks!

**Rob Mercure** · 10-18-2006, 10:28 PM

Hey BD,

Yeah, I was sure something like a wiring glitch was your problem. As to how to use your second input it really depends on what you want to do with the amp. Keeping these pretty much "stock" the Bogen's make really nice "acoustic" amps - the over "sonic signature" seems to match an acoustic guitar well when coupled with a speaker system with some high end response (doesn't have to be anywhere near "hifi" - some guitar speakers do well but you want to get up around 10-12 kHz and many guitar speakers top out around 7-8). And while most folks who use piezo pickups have some sort of preamping the grid leak bias input more closely matches an un-preamped/buffered piezo and you can even up the grid resistor to about 5 M without too much noise - or at least I've been able to.
Now if you're converting the beast for use with a electric guitar you will want to do more modification.
As I noted before, these amps typically run the input stage at only 45 VDC Ep which means PS modification for most any preamps change. If you notice where the "240 V" PS tap goes you've got a 330K and a 470K resistor in series with the 470K Rp for each input stage. One of the reasons Bogen used this scheme was to be able to "mute" input channels "remotely" and grounding either of the "remote" terminals on output terminal strip shorts the B+ for that input stage to ground. While this may seem a tad strange the high value resistors prevent any significant loading of the PS. So to be able to cascade the input stages you'd want to modify at least the "second" stage (old input 2) to cathode bias as the output from the first stage would overload the grid leak bias.
Before I go further - what are you goals with the amp? What sort of music do you play and what guitar/effects are you likely to use? Again, if you're using an acoustic guitar you might just want to leave it alone (although I really like individual output tube bias adjustments, etc.). If not let us know.

Rob

**Blindog** · 10-19-2006, 05:47 AM

I want to use this for electric guitar, so the cascading the two stages together angle is what I'd like to try. I 'm glad you brought up that weird remote circuitry because I'd like to remove all of that if it won't screw up the circuit. I got this thing for a learning tool so I'm open to any suggestions. So would it make sense to make both input stages cathode bias? Thanks for all your input.

Mark

P.S. Should we start a new thread in Mods and Tweaks or will Enzo forgive us?

**Enzo** · 10-19-2006, 07:32 AM

Excuse me? I pay the same membership dues as anyone else here.

A friend recently did one of these and had similar trouble. He had mounted 1/4" speaker jacks on the rear panel right next to input jacks.

**drewl** · 10-20-2006, 02:41 PM

Good thing I gutted one a friend gave me and used it to build a Marshall clone!
After using other Bogens as prototype chassis, it's easier to build something from scratch sometimes!

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