AB2 6F6's - Bogen E-75

[Opcom]

This is a dual channel transformer coupled amp, Bogen E-75, anyone have a schematic for it? I don't see one downloadable on the internet and would like to study it a bit. Not sure it is worth paying for a schematic, rusty old thing may not be usable.

[wa3dsp]

Interesting. There is a site that has lots of Bogen schematics that is free But they don't seem to have the E75.

http://www.schematicsunlimited.com/b/bogen

You probably already found this. I downloaded the E30 which is a pair of PP 6L6'S. Seems strange they are claiming 75 watts from a pair of 6F6's. That is a smaller tube than a 6L6.

This is a good site to reference for schematics in general.

[KB3DKS]

That sounds like the Bogen I have. Complete dual channel output and P.S with a common preamp section. Mine uses 6L6s. One noticible feature is the pair of loctal bias rectifiers. The E-75 schematic is contained in the Sams AA series book AA3 or folder 83-2. My scanner is down right now. If you don't find a copy in a bit can do one for you when that gets back to working.

[Opcom]

I found the Sams folder. Thank you a lot for he offer though.

It has 6L6 outputs, probably AB2, 35W/channel, the 6F6's are class A drivers. All transformer coupled, very nice amp. It's two separate pp 6L6 power amps each with its own power supply driven by a mike mixing arrangement. On the back is a switch to select from which power amp the preamp draws its power. Also has a wide variety of output impedance from 4 to 500 Ohms. I think it will make a nice little amp for SWL receivers and PC speakers. needs a good re-capping. probably all the resistors have changed value as well.

[flintstone mop]

I modified a Bogen J-330 tone control circuits with real bass and treble controls.
The "tone controls" were as broad as a barn and the bass was purposely cut-off at around 100 cycles. Same with the high end. Probably by design, targeted for live PA. use and to reduce feedback.
It is a PA amp with 3 low impedance mic channels and and hi-imp AUX. I use it as a audio monitor for the Diode load output of my 390A.
Fantastic sound with a pair of 6L6's, lashed up to a Radio Shack 3-way speaker box with 12" woofer. Good down to 30HZ.
Fred

[k7mdo]

I have been using a small Peavy amplifier for the diode load monitor on my R-390a but on transmit (even with the receiver muted) the audio feedback from the microphone forces me to turn the Peavy amp volume control down.

I have tried bypassing with some caps with no success... have you had similar problems?

[Opcom]

Any time I had feedback it was a loose mike cable or a tube that didn't have a shield. but I never used a Peavy amp. The chassis of such things - do they have a bottom plate? I remember some that did not.

I scanned the E75 schematic.

The E75 tone control is one of those that is a single pot, it cuts bass at one end and cuts treble at the other. This is OK for PA use. It's smart also because if it cuts and does not boost, moving it around won't overdrive the amp or in case of a speech amp, won't cause more modulation.

Tone is done by C18, R6, and C19. R6 is the pot, and it is tapped in the middle so that there the response is 'flat'.

An interesting feature is how they bias the filaments above ground. The bias is taken from the 6F6's cathode which is at 25V due to the 750 Ohm cathode bias resistor.

The 6L6's can be driven to grid current because the 7Z4 bias rectifier's output is connected directly to the CT of the driver transformer secondary. There is no large resistance.

Interesting voltages are;
6F6
plate +320
K +25V

6L6
plate +430
G2 +340
G1 -34
K -0-

Plate Rectifiers are 5X4, identical to 5U4 except base. Looks like a socket could be wired to take either.