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Nichols 28th May 2006 09:41 PM

Bogen 8417 tube conversion..?
Hi all,
I have a Bogen M120 PA amp which uses 4-8417 output tubes. If it's practical I'd like to convert the amp to use a more common tube. I've read that it's possible to convert these to use 6550's or EL84's, but I've not found anything specific to this amp or the Bogen M Series. There are some references to a conversion on this forum but again, nothing too specific and I'm no electronics journeyman. I have the schematic, I'd be glad to share it with everyone, just email me for a copy. If anyone can help with the conversion it would be very much appreciated. Thanks in advance.

anatech 28th May 2006 09:51 PM

Hi Nichols,
There is some information if you do a search. Burnedfingers was successful I think if you check his thread here

Hey Joe! (burnedfingers), are you looking in?


DigitalJunkie 29th May 2006 03:39 AM


I have a M120 chassis,basically just the chassis and OPT,and was thiking of building an amp around it..The schematic would be Most useful to help figure out all the windings on the OPT!
You've got mail. :)

HF-89 29th May 2006 06:19 AM

Try this link

Nichols 29th May 2006 06:19 AM

RE: 8417 tube conversion
Hi all, thanks for your replies. I'm new to this forum stuff, but I managed (I think) to attach the M120 schematic here. As I said, I'm not electronically savvy, but I know how to use a soldering iron, a multimeter and follow instructions. I looked at burnedfingers' thread and can't make much sense of it, it's above my skill level. Ideally if someone could take a peek at the schematic and say, "okay, do this... replace this resistor here, etc.", I could do that. I've tried to sell the amp for a year (great condition, with tubes) and it apparently has no resale value. If I attempt the conversion and blow the thing up I haven't lost much, so I'm willing to take a crack at it. Any help appreciated, thanks again. -N.

DigitalJunkie 29th May 2006 08:53 AM

Thank you Nichols!

The only real info i've seen is the link that HF-89 has already posted,which is probably a good guide to use for modification.

You'll need to add atleast one potentiometer (preferably 4-one for each tube) ,so you can adjust the bias. It's currently set at a fixed grid voltage. You'll also need to be sure that the bias supply can supply the needed voltage to the grids of the new tubes,you might need to fiddle R45,46,47 a bit to get it right..
Also add a small value resistor in the cathode of each tube,that way you can measure the bias for each tube seperatly.

Hum,some stopper-resistors on the screen-grids might not hurt either.


Nichols 29th May 2006 04:29 PM

Bogen M120 Schematic
I've attempted to upload the schematic but it's too large a file. If anyone would like a copy please email me.

Burnedfingers 29th May 2006 09:06 PM

Actually I believe you should be able to make the swap to 6550's or KT88's and rebias. I don't believe the grid resistors are critical in value but need to be matched. You may have to change the bias pot in order to obtain the correct bias. On the Quicksilver you don't need to parallel the tube sections and I would guess the Bogan's front end will have enough grunt to drive either the 6550's or KT88's to full power.

Burnedfingers 29th May 2006 09:10 PM

2 Attachment(s)
8417 Quicksilver schematic

Burnedfingers 29th May 2006 09:17 PM

2 Attachment(s)
Why not rewire to this?

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