Bogen/Tweed Twin thing

[Smokebreak]

I've got this old green Bogen 50W PA head I've been waiting to use and the time has come.
I'm building a 5D8 Tweed Twin thingy in it, but the Bogen is all octal, so instead of 12AY7 for first 2 stages and CF, I'm using the Bogen's native 6SF5 and for the Twin's 12AX7 PI I'll use the Bogen's 6SL7 in the Paraphase configuration.

I'm about halfway done with the conversion, and am starting to swap out plates and cathodes for the 6SF5s and 6SL7, and while I'm just plugging in the tweed values, it got me thinking "How could one manipulate a 6SF5 to perform as closely to a 12AY7 as possible?"

Same thing goes for the 6SL7, but I think it's closer to a 12AX7..

Any musings?

PS something I just learned is that the HT was coming off pin 2 of the 5U4G of the Bogen?!? Apparently this is possible but with a 5U4GB it must come off pin 8, as usual.

[martin manning]

6SL7 is like a slightly lower gain 12AX7, with 70 mu vs. 12AX7's 100. That will do fine for the PI. 6SF5 is not far from half a 12AX7 with the same 100 mu and very similar ra.

5U4G and GB are both have directly heated cathodes, so it really doesn't matter which pin you take the HT from.

[Smokebreak]

Ok great, thanks Martin. So how would one approximate an AY7 with an AX7? Maybe cut the Rp down to 68K? or split load?

[Smokebreak]

Also, check out the Bogen power supply. There is a 20K resistor the size of a cigar at the screen node. I had omitted it in the tweed build at first, but plates and screens were getting punished at around 490V. I put it back in, at the screen node, and screens came down about 100V.

Is there any reason I can't move that 20K to ground up a stage and put it right at the CT of the OT, to get my plates down to under 400V?

[martin manning]

Ok great, thanks Martin. So how would one approximate an AY7 with an AX7? Maybe cut the Rp down to 68K? or split load?

Split load would work, or maybe better delete the Ck. The 6dB reduction would be just about right, and flat frequency response like the fully-bypassed Twin.

Also, check out the Bogen power supply. There is a 20K resistor the size of a cigar at the screen node. I had omitted it in the tweed build at first, but plates and screens were getting punished at around 490V. I put it back in, at the screen node, and screens came down about 100V.

Is there any reason I can't move that 20K to ground up a stage and put it right at the CT of the OT, to get my plates down to under 400V?

The full HT current might be more that it could handle. Maybe a good time to try a MOSFET B+ reducer?

[Smokebreak]

Thanks Martin, I'm looking into the B+ reduction. Tony is going to send me a board.
After trying out a bunch of things, I ended up just running the 1st 6SF5 at 100K plate and unbypassed 1K5 Rk, and I removed that odd 100K voltage divider that comes off the plate too(This is a single input amp). It served to bring the voltage down below 100V, and I liked it better higher, less muddy. I ended up putting in the later tweed Treble/Bass controls (5E7,etc) and liked the bass control better on that. Paraphase seems fine to me, I've got plenty of cathodynes.
The Bogen has this weird filter set up via a 6 way rotary, so that became my bright switch @ 0,250p,500p,1000p,2200p,4700p and is a GREAT addition here.
Great sounding amp, gets grindy cranked but not over the top. I started adding .68 caps and 470p peakers and you can quickly see where things are headed, historically speaking