Anyone Built This Valve Preamp?
- Thread starter Vikki
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Buttercup
Well-known member
What happenned to Fred?
He has a nice site.
He has a nice site.
> wondering if its worth having a go, i thought a couple of channels for my acoustic guitar.
He never tried the instrument amp version.
There is some strange design there. Looking over the site, it makes more sense in the context of the hi-fi "reference amp" that it is adapted from. As an instrument amp, it seems a little complicated without obvious reason. He went to great length to direct-couple the signal stages, and ended up with a lot more capacitors than a simple cap-coupled amp.
Are you an experienced constructor? "Somebody" should build this amp and report on it. I think it will generally work fine.
V2B cathode lives at +250V, technically over the heater insulation rating of the tube. It will probably work with 9 of 10 tubes at first, but there could be heater shorts after many hours of running. There are several ways to deal with this. Since both halves of V2 can live with AC heat, a 6VAC transformer just for V2 is one way. Changing the ground on the proposed heater supply to a +100VDC line might be another way to split the difference and stay inside ratings.
The 100K input resistor is a little low for some pickups; 470K will work just the same.
Volume control at output means possible high output impedance, and possible (unlikely) overload on extremely hot pickups. We can't set the gain in the middle of the amp: feedback. A volume pot at the input is usually acceptable for pickup uses.
I'm puzzled by the Ctwk cap. I don't think it corrects for cable loading as he suggests. It may correct for stray capacitance in the wires to S1, RV1-3, and V1 pin 7. If so, a tight layout may be better than a tweak.
The V2B cathode follower is added from the hi-fi version. My instinct is to take all feedback from V2 pin 8 instead of V2 pin 1. If he had prototyped this amp, I would not argue if he liked it better this way. It would be simple to add a SPDT switch to try it either way while playing. There will be a 1V pop when switched: startling but not likely to damage speakers or ears. (That assumes a snap-action switch. A slow switch would leave the amp without DC or AC feedback while switching, major pop.)
The AC feedback is so effective that this may not have "tube sound". I've built stuff like this and it wasn't very tube-y.
If you are not looking for electronic adventure: given two twin-triodes, you could just knock out a pair of buffered gain stages and put a passive tone control network between them or a feedback tone net around the output stage. A Fisher or Scott hi-fi amp would do this chore in one twin-triode, not two, and be more tube-y.
He never tried the instrument amp version.
There is some strange design there. Looking over the site, it makes more sense in the context of the hi-fi "reference amp" that it is adapted from. As an instrument amp, it seems a little complicated without obvious reason. He went to great length to direct-couple the signal stages, and ended up with a lot more capacitors than a simple cap-coupled amp.
Are you an experienced constructor? "Somebody" should build this amp and report on it. I think it will generally work fine.
V2B cathode lives at +250V, technically over the heater insulation rating of the tube. It will probably work with 9 of 10 tubes at first, but there could be heater shorts after many hours of running. There are several ways to deal with this. Since both halves of V2 can live with AC heat, a 6VAC transformer just for V2 is one way. Changing the ground on the proposed heater supply to a +100VDC line might be another way to split the difference and stay inside ratings.
The 100K input resistor is a little low for some pickups; 470K will work just the same.
Volume control at output means possible high output impedance, and possible (unlikely) overload on extremely hot pickups. We can't set the gain in the middle of the amp: feedback. A volume pot at the input is usually acceptable for pickup uses.
I'm puzzled by the Ctwk cap. I don't think it corrects for cable loading as he suggests. It may correct for stray capacitance in the wires to S1, RV1-3, and V1 pin 7. If so, a tight layout may be better than a tweak.
The V2B cathode follower is added from the hi-fi version. My instinct is to take all feedback from V2 pin 8 instead of V2 pin 1. If he had prototyped this amp, I would not argue if he liked it better this way. It would be simple to add a SPDT switch to try it either way while playing. There will be a 1V pop when switched: startling but not likely to damage speakers or ears. (That assumes a snap-action switch. A slow switch would leave the amp without DC or AC feedback while switching, major pop.)
The AC feedback is so effective that this may not have "tube sound". I've built stuff like this and it wasn't very tube-y.
If you are not looking for electronic adventure: given two twin-triodes, you could just knock out a pair of buffered gain stages and put a passive tone control network between them or a feedback tone net around the output stage. A Fisher or Scott hi-fi amp would do this chore in one twin-triode, not two, and be more tube-y.
Kev
Well-known member
[quote author="mcs"]He died of cancer late September last year.[/quote]
:shock:
...
was I completely asleep when this happened ?
this is very sad news to me
It doesn't seem that long ago but Fred and I exchanged some emails in the past and although he didn't frequent Tech Talk I'm sure he did drop in once or twice. I still have a Fred project on the bench with all parts ready to go but no time.
:shock:
...
was I completely asleep when this happened ?
this is very sad news to me
It doesn't seem that long ago but Fred and I exchanged some emails in the past and although he didn't frequent Tech Talk I'm sure he did drop in once or twice. I still have a Fred project on the bench with all parts ready to go but no time.
Fred's passing is news (and a shock) to me as well. I never communicated directly with him, but I built one of his 1 watt tube amps using the 13EM7, & it gets regular use for recording guitars & keyboards.
What sad news.
Tom
What sad news.
Tom
clintrubber
Well-known member
This is sad news indeed - it doesn't feel to well when we know that people around here have had to leave... may his spirit live on in those circuits...
Peter
Peter
ciminosound
Well-known member
Wow, I've been gone too long. I trade some emails with Fred a few years back before I found tech talk and he was a great help to me.......
