Help adding a gain stage to the Valco aka Supro Thunderbolt

[smackoj]

Hi; I have a Gretsch 'Pro Bass' version of this Valco amp. It's the same amp except the Gretsch added another input jack to make a total of 3. I love the tone but would like to experiment with adding more gain. I'm not looking for a lot of overdrive or breakup sounds, just interested to know what using the other half of V1 could do for it?  Thanks amigos

http://www.el34world.com/charts/Schematics/files/supro/Supro_6420.pdf

Jack D

 

[Jack_Hester]

Jack -

A couple ways to use it come to mind.  You can slip the second half in right after the volume pot, and just before the tone circuit.  The first way would be to use it purely as another gain stage, and wire it very similar to the first stage.  The second would be to wire it as a cathode follower.  You can model it after a circuit like the 5F6-A Bassman CF.  Either way, both would fit in with little or no mods to the chassis.  The CF would not be a gain stage, as it would only have a gain of something approaching 1.  But, it would provide an impedance match between the first stage and the tone circuit.  Should improve the signal going to the phase inverter, by taking the load created by the tone circuit off of the first stage.  You'll also hear some (maybe a lot of) improvement in the tone control. 

I'm sure that others will speak up and give you more options. 

Jack

[tubenit]

Since you are asking about added gain, I am presuming you're wanting to use this for guitar not bass?  Correct?

NOTE that this amp's preamp is for a BASS guitar and and it filters out much of the highs and is purposely very very low gain.
From the input jack to the first triode it drops gain and emphasizes lower frequencies.

Note that this would require 3 pots for the tone stack .......... or you could have a fixed resistor value for the mid pot and have two pots for
the  bass and treble.   In fact, if you had a fixed resistor for the "mid pot", then you could actually be creative and use a dual gang 500k pot for these tone stacks.

You could do what Jack Hester said and use a mosfet CF to make a psuedo 5F6-A.

OR you could just use both triodes and have a Dumblish style tone stack with a PAB (pre amp boost) and midboost. You can get some pretty decent gain with a "fat" tone using this.

You can obviously substitute a different type of tone stack on either of these ideas.

IF you are interested in either of these designs & need help with a layout, I can provide one.

With respect, Tubenit

[tubenit]

A couple more ideas.   IF you're NOT willing to have 3 pots for your tone stack and want to use just one, then you could use either of these.

The James tone stack can be very easily built on a dual gang 500k pot.

Tubenit

[smackoj]

A definite yes to the question of using this amp for guitar and not bass! Thanks so much for your help. I like your ideas about improving the tonal quality and not trying to overdrive the tubes.....muchas gracias

Jack D

re: Jack Hester: love the George Orwell quote

[smackoj]

"IF you are interested in either of these designs & need help with a layout, I can provide one."

Tubenit; I like the look of the 2nd schem you offered "added gain stage with PAB and mid boost"
would you mind sending along a layout if it's not too much trouble?

Thanks again JH and Tubenit for the great advice.   

Jack D

[Willabe]

I don't see how this controll will work. It seems no matter where you set it the wiper, because it's tied to 1 end of the pot will always get the full signal from that point?

I don't see how it would get any signal from the 500pF coupling cap even if rotated all the way to it?

I can see if you rotate the pot all the way to the junction of the pot and 10K R you will lose signal to ground. But what does the 500pF cap do?

Is it just a high end bleeder/shunt limited by the 10K to ground? So it has nothing to do with the 250K 500K pot?


              Brad      



          

[smackoj]

I don't see how this controll will work. It seems no matter where you set it the wiper, because it's tied to 1 end of the pot will always get the full signal from that point?

Perhaps I chose the wrong one? It appears that I did.  I perhaps should try the "CBS-JTS.gif"   one instead?

thanks again one and all.

Jack D

[Willabe]

No I think I answered my own question. I just misunderstood how it worked.

You picked a great one.


             Brad     

[tubenit]

Tubenit; I like the look of the 2nd schem you offered "added gain stage with PAB and mid boost"  would you mind sending along a layout if it's not too much trouble?

Here you go.  If the values don't match the schematic, change the layout values to match the schematic.

The cathodes and the 4.7M are connected to a buss wire soldered across the back of the pots.  You can see Hoffman's Library of Information for an illustration of that.

With respect, Tubenit

[tubeswell]

You could also use the spare triode for a switchable parallel-single V1 stage

[smackoj]

what does having the two triodes in parallel do? I've never seen that before and couldn't say if I ever heard it?

[tubenit]

Parallel triodes increase gain by around 30% with no increase in floor/idle noise.

Tubenit

[tubeswell]

what does having the two triodes in parallel do? I've never seen that before and couldn't say if I ever heard it?

What tubenit said.

You have the choice of a 'hi-gain' or ('fat') input or a 'normal-gain' (or 'clean') input. For the same HT voltage, the paralleled stages result in twice as much current through the single plate resistor, and for a slightly higher bias voltage, than the single stage.  This equates to a bigger voltage drop (and a bigger voltage swing for a given amount of input signal vac) across the plate resistor, which equals more gain (as well as a cleaner signal - because of the higher bias voltage). If used in v1 (as above), you get a better signal:noise ratio throughout the whole amplifier signal chain, which is helpful for tone (because the lower the SN ratio in V1 the better overall amplification), which is great for studio amps.

In addition, the output impedance of the parallel stage is lower than the single stage, so you preserve more frequency bandwidth through the following load, than you would with the single stage - so a parallel stage drives tone stacks better.

[smackoj]

re: "You have the choice of a 'hi-gain' or ('fat') input or a 'normal-gain' (or 'clean') input." etc.

thanks much amigos

[SoundmasterG]

The parallel triode also has a fat sound which loses some of the high chime of say a blackface Fender, but it is very rich harmonically in comparison. Some say a gravelly sound....

I like them myself...

Greg

[sluckey]

what does having the two triodes in parallel do? I've never seen that before and couldn't say if I ever heard it?

Some (not all) of the popular Marshall 18 watt use parallel triodes when connected thru the normal channel HI input.

[smackoj]

Is it a reasonable idea to add any pots or switches on somewhere other than the face of the orig. amp (my T-bolt is from '67)? I'm not thinking I would sell it but never say never so would like to avoid drilling holes in the face. Possibly using a piece of chassis material to mount the new stuff then link it together to the T's chassis with shielded lines?

thanks in adv.  Jack D

[sluckey]

Have you tried any of the suggestions yet? I'd try the easiest first. It would take 5 minutes to do the parallel triode idea and 5 minutes to undo the mod without leaving any scars. I personally would not use this amp as an experiment.

[smackoj]

I have the first triode values changed (for parallel gain) but I'm not quite sure how to attach the 2nd side so it's only on when the #2 input is being used? I don't read schem very well yet.  thanks,  Jack D

[sluckey]

Don't change any component values. Just connect a 1/2" jumper between pins 1 and 6, another jumper between pins 2 and 7, and a third jumper between pins 3 and 8. If you don't like the fatter sound simply remove the 3 jumpers. If you like it, let us know and we'll help you get it set up for selectable use with the input jacks.

[smackoj]

10-4 Sluckey, I'm on it.  gracias

[smackoj]

ok, both sides of the 12AX are parallel and the gain is much better. I like it mucho but will have to try it with some other speaker combos I use alternately and get the full picture. But, very nice and plenty of clean still on tap which is exactly what I wanted! Excellente       

[sluckey]

Play with it for a couple days. If you still like it and want to do the hi/lo input jacks, let us know. It's easy to do. However, if you like the fatter sound I bet you'll never use the thinner sounding lo jack.

[smackoj]

si senor. muchas gracias y hasta luego.  Jack D