Building a Funnpeg (Fender/Sunn/Ampeg in one amp)

[birt]

I decided to build a bass amp on the chassis of my Super Twin Reverb. it's been collecting dust and if i'd sell it i can't really get more than the prices of the tubes and speakers...

since this amp is an absolute mess inside i will take out most of it. the heater wires aren't properly twisted, wires are way too long etc. it's a miracle those thing work without much hum and noise.

i will keep the poweramp board and most of the power supply.
it will have a Sunn 2000S preamp, an Ampeg V4B preamp and a Fender Studio Bass preamp (minus the 5 band EQ).
i'm thinking one input to all 3 preamps and a footswitch to select one or more parallel. each also with a seperate input that cuts it from the shared input. the fender preamp will be out of phase with the other 2 though.

right now i'm sketching the preamp layout on paper. i have the poweramp and PSU attached to this post.

since this is quite a beast i decided to add some extra failsafe things.
i added HT an OT fuses but i don't have the values figured out yet. i was also thinking about a circuit that shuts down the HT when there is something wrong with the bias supply. but if a tube starts pulling too much current the OT fuse should melt right?

i also elevated the heaters to about 80V.
all voltages indicated on the schematic are from the super twin reverb schematic and should be correct. only the voltages at points A, B and C are a bit of a guess. they are what i need for the different preamps. (A:Ampeg, B:Sunn, C:Fender)

[PRR]

Looking at the PNG drawing:

The 39K 2W resistors dissipate 1.6W. That's way too close for comfort, considering how wall-voltages may run high (12% high puts these parts over rating). Use two 75K 2W at each 39K 2W position.

In fact since the 250V:250V split is enforced by the PT CT, and you have that 30K (20W!!) bleeder, I don't see what the 39K bleeders add.

One of your bias supply caps is shown in wrong polarity.

And I'd use 100V caps there.

IMHO, the main bias-failure mode is that stupid "balance" pot. WHEN (not if) the wiper lifts you lose all bias. It also has an awful high current for a pot, and is not a readily-available part. And you can't adjust tube current, only trim the balance to null plate-buzz (which you could do by swapping tubes from side to side, and discarding any way-out tubes). I'd steal a bias-plan from some other amp, only scale to lower impedance because we have six grids to hang onto. Look for a 5K _2W_ mil-spec pot, wire as reostat, put 5K under it (to ground) so you can't turn bias down to zero.

Did the original use a shorting speaker jack? My gut says that is a bad idea on this beast, but I should yield to CBS's wisdom.

The 12AT7 cathode voltages look dubious. 70V? Then with 33K DC load and another 33K AC load they can only pull-down 35V. But the bias is near 60V. Usually your pull-down shoud somewhat exceed bias. Did this work for Fender?

[birt]

I've said you can mix-and-match preamps with power amps.

However you have found the three extremes.

The 1971 Ampegs take a lot of gain, drop down through tone networks, and have a high gain power amp. About 0.2V into the power amp.

That Fender has gain, tone, gain into a fairly low-gain power section. About 1.4V into the power amp.

I don't have your Sunn plan to hand. Early Sunn had high-gain power stages.

This is a major re-design project.

You will be re-discovering many design decisions made by three different designers.

And I'm not sure they came to VERY different results. Aside from the obviously different tone stacks. I applaud skipping Fender's not-graphic 5-band EQ, but the Ampeg has the same thing as a mid-only control, and impemented very complexly.

Quoted from my other topic for convenience.
i found the schematics here:
http://www.schematicheaven.com/ampeg.htm
http://www.schematicheaven.com/newamps/sunn_2000s.pdf
http://www.schematicheaven.com/fenderamps/studio_bass_200w_schem.pdf
and the donor amp:
http://www.schematicheaven.com/fenderamps/super_twin_reverb_180w_schem.pdf

i'm still not sure about the ampeg part. i have played the SVT classic and the B25b and liked them both a lot. so i was searching for something in between and thought the V4b would get me there. other suggestions are very welcome. if i choose another one i should do it now before i start doing the layout.

[birt]

i made the corrections you mentioned (i hope i did the bias right?) on the schematic.

then i measured some voltages on the super twin (i haven't done anything to it yet), they are in blue on the schematic. there were no output tubes in the amp.

[The_Gaz]

PRR was also saying that the 'current balancing' resistors are redundant when using the PT's CT. It's easier and more economical to just add one bleeder to screen filter - it will bleed of the voltage the same.

[birt]

if i want to use the V4b preamp i think i'll have to make V3a (last triode in the preamplifier schematic) a proper gain stage instead of a cathode follower and then take the signal from the anode.

i was thinking 100K plate resistor, 3K cathode resistor, 1m grid leak resistor. and a cathode bypass cap. considering it will be a 12AU7 triode the cathode should sit near 6Vdc. i think that would be a gainstage with an amplification factor of  18. i hope i calculated right :p.
if the last gainstage is also a 12AU7 (triode half of the 6AN8 pentode in the Sunn preamp) that would give me an output of more than 1V instead of 0,2v i think. so it gets closer to the fender preamp.

i can't really figure out what output of the Sunn preamp would be right after the pentode though.

[birt]

i made a first drawing of the preamp. now i need people to tell me i did it all wrong :p

first one is the Fender Studio Bass preamp with bypass cap on the second triode cathoderesistor. i think the 750µF cathode bypass caps were unnessesary and not really a common value so i changed them.

second one is the Sunn 2000s preamp. i think it will have enough gain. i removed the feedback at the pentode cathode and added an output cap.

third one is something inbetween the Ampeg V4B, B25B and the third gainstage is something i did. i sure hope it works as i intended.

i also used both inputs of the PI so all preamps are in phase. problem is i can't insert feedback there anymore. any ideas?

[sluckey]

second one is the Sunn 2000s preamp. i think it will have enough gain. i removed the feedback at the pentode cathode and added an output cap.

I've always considered that 6AN8 pentode stage as part of the PI since the orig Sunn split load PI has no gain. And now you're driving a LTP PI. I think you're gonna have way too much gain. I'd drop the 6AN8 and take the Sunn preamp out directly from the treble pot. Then the Sunn and Fender channels would have similar gains.

A bass amp will want some negative feedback. Do you have plans?

[birt]

i kept the pentode because i was thinking it might be a part of the preamps tonal character. if i want to keep it i could maybe take the signal from the cathode? or make that master 100K and maybe add some more attenuation there?

i should probably just mix all 3 channels and use the feedback as it was before, at the PI?
also i noticed i don't need those .022 caps before the PI

[sluckey]

i kept the pentode because i was thinking it might be a part of the preamps tonal character. if i want to keep it i could maybe take the signal from the cathode? or make that master 100K and maybe add some more attenuation there?

That Sunn power amp (including the 6AN8) is a direct copy of an old Dynaco HI FI amp. It's all about faithfully reproducing the preamp signal with minimum coloration, distortion, or tonal change.

i should probably just mix all 3 channels and use the feedback as it was before, at the PI?

That's what I would do. In fact, I'd mix all channels into a single Master Volume, then feed one side of the PI.

also i noticed i don't need those .022 caps before the PI

Those caps are very necessary with that LTP PI. They prevent the preamp circuits from loading and disturbing the dc bias on the very high impedance bootstrapped inputs of the LTP.

[birt]

so there is not much pentode tone there? if there is any at all... so if i take the signal right after the tonestack i can take out the 6AN8. but that means i need to add a 12AU7 for my third triode in the Ampeg-like preamp. so i would have an extra triode. is there any place in the amp that would benefit from a cathode follower? if not i might parallel it for that gainstage.

i mixed them all but after their seperate master volumes. the reason for this is that i want to be able to use them completely seperately for keys or other electronic toys. that way i can balance the volumes. NFB is at the PI again as it was.

i'll add the .022 cap again.

thanks for the input!

[HotBluePlates]

I've always considered that 6AN8 pentode stage as part of the PI since the orig Sunn split load PI has no gain.

so there is not much pentode tone there? if there is any at all...

I agree; the pentode is an integral part of the inverter, just like the stage ahead of all split-load inverters.

The triode section of the 6AN8 is essentially 1/2 of a 12AU7. That's a good low plate impedance for driving 4x 6550's. But you need 55v peak output, or about 110v peak to peak, with a stage gain of less than 1. Few preamps output 110v peak to peak, more often a couple volts maximum.

The pentode half of the 6AN8 is there to Make the split-load easy for the preamp to drive, which is the same situation every time a split-load inverter is used. The pentode allows a couple-volts from the preamp to make the big input needed for the split-load.

[birt]

so right now i'm happy with the fender and sunn.

any comments on the ampeg-like preamp?