power supply question

[ernie_jr]

I am building a Sunn 200s with a second pre-amp. The current power supply has a 33k resistor feeding the pentode plate of the PI and both plates of the pre-amp. If I add in a second pre-amp should that resistor become a 22K?Thanks in advance,Ernie

[shooter]

fwiw
it's not a good idea to power multiple gain stages from 1 tap, a kinda good rule to follow is no more that 2 gain stages per tap (resistor-cap).
I would add a complete "new" parallel PS "rail" for the 2nd preamp

[sluckey]

I suggest adding a separate node D just for the extra preamp. Use a 33K/1W and 20µF. Connect the 33K to node B.

You know the 200 is a bass amp, right? If you want a simple guitar friendly preamp look at the Spectrum II. Same amp but with a guitar preamp. Heck, build both and cover bass and guitar.

     https://el34world.com/charts/Schematics/files/Sunn/Sunn_spectrum_ii.pdf

[ernie_jr]

thanks Slukey, it is only a bass amp, sunn pre-amp and an ampeg B 25 pre-ampThanks,Ernie

[ernie_jr]

OK, so finally finished this build, Basically a SUNN 200s with a second preamp from an Ampeg B25, Running solid state rectifier and KT120 tubes.All is well and it sounds great. Only issue is the Ampeg I can turn it up to 10 and get it loud. The Sunn rattles the house at 3. Is this normal.All voltages are within 10% of spec. I settled on a 22k resistor for the sunn preamp off is own B+ node, giving 250 volts unloaded, specs call for 210.The Ampeg is 250 unloaded, specs call for 245. Not bad considering unloaded B+ is 280 volts. Will post pics soon. Any ideas on why Ampeg preamp does nnot drive the 6an8 as much would be appreciated.Thanks for all the help thru out this build.Ernie

[ernie_jr]

Also, want to say this is built off a Hoffman board and it is dead quiet, no hum at all. Thanks to Doug for making a killer board that fit perfectly. Power supply is solid state with 110, 50, 20, 20, 20 mfd filtering for the stages.Ernie

[sluckey]

OK, so finally finished this build, Basically a SUNN 200s with a second preamp from an Ampeg B25, Running solid state rectifier and KT120 tubes.All is well and it sounds great. Only issue is the Ampeg I can turn it up to 10 and get it loud. The Sunn rattles the house at 3. Is this normal.All voltages are within 10% of spec. I settled on a 22k resistor for the sunn preamp off is own B+ node, giving 250 volts unloaded, specs call for 210.The Ampeg is 250 unloaded, specs call for 245. Not bad considering unloaded B+ is 280 volts. Will post pics soon. Any ideas on why Ampeg preamp does nnot drive the 6an8 as much would be appreciated.Thanks for all the help thru out this build.Ernie

Could be normal. Hard to guess without a schematic of your work.

[ernie_jr]

It is just the preamp of the B25 grafted into the PI of the Sunn

[sluckey]

Let's see the graft. 

[ernie_jr]

here is are the pics. the Ampeg feeds into the 6an8 at the same point as the Sunn preamp

[sluckey]

You need to use two mixing resistors. The wiper of each treble pot needs to connect to a 220K or 270K resistor. Then connect the other end of those resistors together. And finally connect that junction to the 6AN8 grid pin 8. Is this what you have?

[ernie_jr]

Only have a 270k on the ampeg, the Sunn feeds straight in, just as in the schematic

[sluckey]

Only have a 270k on the ampeg, the Sunn feeds straight in, just as in the schematic

So, fix that and see if the volumes are more compatable.

[PRR]

> the Sunn feeds straight in, just as in the schematic

The Sunn is a one-channel amp. You want a two-channel amp? You may need a mixer.

What Sluckey says.

However, the Ampeg is to lower-gain stages and a high-loss tone network, the Sunn is two high-gain stages and a lower-loss tone network. I think the Sunn output at this point is ten times the Ampeg.

[Willabe]

How do you like the KT120's?

Do you use a bigger PT and OT for the KT120's?  Or the same PT/OT that you normally use in a Sunn 200s?

[DummyLoad]

for the B25 preamp - specifically the james stack driver triode, add a 47uF bypass cap around that 3.3KΩ cathode resistor. that should get the gain structure of both preamps closer to each other. optionally, make it switchable to regain tone structure of the original. see attached for depiction. 



--pete

[ernie_jr]

Thanks for all the advice. Will put in a 270K on the Sunn and a 47uf on the Ampeg. As for the KT120s. They sound great, lots of head room. Using the stockSunn tranny as it has plenty of amps for the filaments and since not using the 5 volt tap, a little less strain on the tranny.Ernie

[Willabe]

Thanks for all the advice. Will put in a 270K on the Sunn and a 47uf on the Ampeg. As for the KT120s. They sound great, lots of head room. Using the stockSunn tranny as it has plenty of amps for the filaments and since not using the 5 volt tap, a little less strain on the tranny.

What about the OT?

[ernie_jr]

Both trannys are stock, The O/P if one of the original Dynaco transformers, the power was the second generation version. It still runs cool with the KT120s.
Ernie

[Willabe]

Thanks Ernie.