5F6 preamp voltages

[Bub]

Hi Gentlemen,


I built a KOC 5F6 Bassman out of TUT 3 a few years back and then had to leave it alone for a few years so I'm trying to pickup where I left off.
I  put the KOC (Best All-Tube effects loop) in, using a 12AT7 on the same B+ rail as the as the preamp tubes which I found pulls the over-all preamp voltage down a bit so I know I need to adjust the dropping resister to compensate. I'll do that later.
 The Hammond 272JX PT has two taps, one for 115 VAC and one for 125 VAC. When I built it I used the 125 VAC tap and the voltages were a little low.  I have changed to the 115 VAC tap to get the voltages up a bit closer to original specs.
I just need to know if voltages are in the ballpark of where they need to be.
The voltages is with V3 pulled which is the effects loop 12AT7.
V1 12AX7, V2 12AX7, V3 12AT7, V4 12AX7, V5 & V6 6L6GC I will post a schematic.


V1 Pin   1          2          3          6          7          8
           167        0        1.27     164        0        1.3


V2 Pin    1         2          3          6          7          8
            247     140      141      140        0         .9


V3 pulled


V4 PI Pin    1          2          3          6          7          8
                194       23        36       207       22        36


V5 & V6  Pin    3          4          5          6          8
                      414      405      -36       406       0


Thanks for any help
Rob


             

[Bub]

Schematics

[sluckey]

Voltages look OK to me. How does it sound?

[Bub]

It sounds great but I was expecting a little more output before clipping I see on my scope.
Just before clipping I measured 11 VAC at the speaker jack into a 4 ohm, 100 watt load.
I guess I'm trying to find a little more clean headroom.
I put a set of 6550s in biased at 70mA and it pulled plate voltage down to 400v, preamp voltages stayed the same.
It does sound better. I measured 14 VAC at the speaker jack just before clipping
I wasn't quit sure of what voltages I should see on V2


Thanks


Rob 

[PRR]

I don't see this effects-loop; not even a place for it.

If you are looking for headroom, you know that low B+ suggests smaller dropping resistors.

14V in 4 Ohms is 50 Watts. Not unlikely for 400V and 3.4K load.

Real Bassman with 400V and 4K load is 40 Watts.

Up-sizing the tubes (5881 to 6550) won't increase output on the same load by very much.

The lower load is good for some more watts (if the tubes will pull it), and (4K/3.2K)*40W is just about 50 Watts.

V2 is an elaborated 5F6a Bassman tone driver. Which is a modded Standard Fender Stage plus a cathode follower. (The whole amp is based on 5F6a.) (Ha, it even says Bassman.) The SFS with 1.5K cathode resistor sets the plate about 69% of the supply. That's too high for a direct-coupled cathode follower, so Leo switched to 820r cathode resistor. This should bias plate nearly half the supply. 247V, 140V, that's half-enough for our purpose. And should be good for 50V peak swing into the tonestack. Taking TS loss as 10:1, that's 5V peak into the driver. The whole driver/output stage seems to work at gain of 12, or 8, possibly 5. So 5V out of tonestack "should" give 25V peak or 18Vrms at the speaker. With this B+ and load, the output stage won't do over 14V. So the stuff before it is not really restricting the output.

However 18V is not a lot of headroom over 14V.

The 247V to V2 sounds low. I thot Leo ran above 300V here.

For "MORE", a solid-state rectifier is 10%-20% more Watts output.

If that's still less than you need, double everything (100 Watt parts) and use JBL or E-V "lead guitar" speakers. Or a Full Stack (8) of hot Tens. (There's more than 2:1 difference of efficiency between "loud" speakers, and at this scale a high efficiency speaker (or array!) is better leverage than doubling the amplifier power.

[Bub]

Thanks for the reply.


My back can't handle 8 tens anymore. I have a Ampeg 410 ported bass cab which is more than enough. The gang from 40 years ago get together for a basement jam about twice a year. Other than that I'm just annoying the neighbors.
I do have one other amp I built which has 4 6550s so I did the double everything.
I see Leo had 325v on V2 but with all the mods from original I wasn't sure where the voltages needed to be.
You have me thinking about how I have wired my effects loop in and if it is correct. I installed it between the TS and the PI.
It works, but as other people have said it seems sterile. I installed what I would call a true bypass switch to remove it from the signal path.
It certainly alters the speaker output scope pattern. I will post pics but I have to wait until the weekend.