Transistor Headroom...

[petemoore]

I want a simple preamp to drive a Dynakit ST70.
  It has a pentode configuration 7199 tube before the output tubes, they step up from what many pre-amps put out [I understand], I'm thinking I might get something I like in a Transistor or OA build.
 I've never tried higher voltage ccts, more headroom would be available with them.
 Mosfet Booster gets the thing going pretty good, But doesn't drive it as hard as the Behringer mixer, which has plenty more than enough output to drive the Dyna.
 That said, is there somethign fairly simple I could try with say Dual OA's or Q's?

[brett]

Hi Pete
I'm no expert about driving what you've got, but I would have thought that with that pentode there, all you'd need was something that would push a volt or two (RMS) into it.  So something like a rail-to-rail op amp like the NE5532 would be good, even on a single 9 or 15V supply.  

For really huge output (30V p-p!), use this op-amp on a +/- 15V supply.  My amp is a similar setup (I think) and uses a crappy solid state front end (LM307s from about 1975!).  Even this front end, on a +/- 15V supply can massively overdrive the subsequent 12AX7 and EL34s.

cheers

[will]

Hi Pete,

Mosfet Booster gets the thing going pretty good, But doesn't drive it as hard as the Behringer mixer, which has plenty more than enough output to drive the Dyna.
 That said, is there somethign fairly simple I could try with say Dual OA's or Q's?

How about 2 Mosfet booster stages. That will have enough gain to overdrive the Dyna ST70.   :twisted: You could also look into putting a tube rectifier in the ST70.

Regards,
Will

[petemoore]

Kool Idea about the Mosfet Boosts.
 How high of a supply voltage can I go on a Mosfet Boost? I don't know that supply voltage is the answer either, but it seems like I would get the swing to happen at higher voltages that way.
 The ST70 has a GZ34 rectifier tube.

[will]

Hi Pete,

Kool Idea about the Mosfet Boosts.
 How high of a supply voltage can I go on a Mosfet Boost? I don't know that supply voltage is the answer either, but it seems like I would get the swing to happen at higher voltages that way.
 The ST70 has a GZ34 rectifier tube.

The max voltage for a Mosfet depends on which one you are using, check the datasheet. My understanding for High-Fi equipment; an unwritten guideline was 2volt input to power amp gives max power before clipping.

I’m not exactly clear on what you want to achieve with the ST70. Do you want to make it a standalone guitar amp? If so, you do need a preamp section, you could use one of your existing effects like a meteor, Vulcan, Thunderchief or a highway 89 could work well. Or put in one or two 12ax7(s) and build a true tube preamp into the chassis.

Another option is just to keep it essentially as is perhaps increasing the gain of the circuit and just use it with your effects chain with just a volume control.

Good luck with your venture.

I would like to find ST70 for one of my stereos, with some updates and upgrades it can be a good sounding amp.

Regards,
Will

[petemoore]

St70's 'work by themselves, at a fraction of the possible output available when driven by a preamp/
 That's the way I've been using it, for testing jamming and recording, a simple boost or fuzz is enough to make it kinda loud.
 But I want to try makeing it 'loud' again,...since the Dyna stereo tube preamp died, and was decimated, it hasn't seen much high input levels.
 Really all I think I want for now is something simple that will be like a channel of a board's output level, maybe a volume control.