I'm planning to put a VVR (Hall-type) in my current EL84 SE amp project (ADA Rocket 10).
I can choose to control the whole amp, output tube screen only or plate and screen, as the wiring is easily accessible.
It seems that lowering the power tube voltage only would make the output section even more overdriven (by the preamp stages running flat-out) than at present. I want to keep some clean sounds and just lower the volume level at the speaker, so I'm not sure that regulating only the power tube is the best option. But, will lowering the preamp voltages lead to more distortion as well?
Your experiences and/or recommendations would be appreciated!
Thanks
I can choose to control the whole amp, output tube screen only or plate and screen, as the wiring is easily accessible.
It seems that lowering the power tube voltage only would make the output section even more overdriven (by the preamp stages running flat-out) than at present. I want to keep some clean sounds and just lower the volume level at the speaker, so I'm not sure that regulating only the power tube is the best option. But, will lowering the preamp voltages lead to more distortion as well?
Your experiences and/or recommendations would be appreciated!
Thanks
I've implemented VVR in a push-pull amp, never a single-ended one. In that case, I scaled only the PI and output section. It worked rather well, the goal being more distortion at a lower volume. I've also had experience with a '65 Amp with a master volume that was a VVR control, scaling the whole amp. I didn't care for this implementation as the amp got quite mushy when used.
How this all translates to SE I'm not sure but for clean tones at lower volume a simple volume control would seem to be best. My advice would be to try your different implementations and see which, if any, work best.
How this all translates to SE I'm not sure but for clean tones at lower volume a simple volume control would seem to be best. My advice would be to try your different implementations and see which, if any, work best.
Thanks, unique1.
Update:
I did try a couple of different ways to insert the VVR in the amp. Since the Rocket 10 overdrives the output tube quite readily, and doesn't have a master volume right before the output tube, it really got overdriven much too easily when I used the VVR on the output tube plate and screen only. Reading online indicated a MVol would be needed to make that work, and the faceplate for the amp was already made.
So I moved a wire on the eyelet board and 'scaled' the whole amp which works much better in this case. Obviously it varies with different amps and with the goal (more, or less, distortion for example), since your experience was the opposite.
The VVR in this amp is a much better 'tone and overdrive controller' than it is a 'volume and loudness control' which is OK with me. I don't think VVR will take a 10-watt amp down to 1/4 watt for 'bedroom' use without very dramatic tonal changes, but that may just be this particular amp.
Thanks again for your input. I realized after posting here that probably there were better places on the web to look for info on VVR - in particular the VVR sub forum at wattkins, where I did some reading.
Update:
I did try a couple of different ways to insert the VVR in the amp. Since the Rocket 10 overdrives the output tube quite readily, and doesn't have a master volume right before the output tube, it really got overdriven much too easily when I used the VVR on the output tube plate and screen only. Reading online indicated a MVol would be needed to make that work, and the faceplate for the amp was already made.
So I moved a wire on the eyelet board and 'scaled' the whole amp which works much better in this case. Obviously it varies with different amps and with the goal (more, or less, distortion for example), since your experience was the opposite.
The VVR in this amp is a much better 'tone and overdrive controller' than it is a 'volume and loudness control' which is OK with me. I don't think VVR will take a 10-watt amp down to 1/4 watt for 'bedroom' use without very dramatic tonal changes, but that may just be this particular amp.
Thanks again for your input. I realized after posting here that probably there were better places on the web to look for info on VVR - in particular the VVR sub forum at wattkins, where I did some reading.
Hi, I put VVR in a recent design. A push-pull amp. The VVR is only active on the output stage. Works fine. A secondary benefit is that because it uses octal output tubes (KT88 standard in this case) you can use other less powerful tubes if the have they same base at reduced voltages. The output transformer will not perfectly match some, but is close enough to work satisfactorily. The mis-match will mostly effect power efficiency and the sound is fine. Any tube from a 6V6GT to KT88 including EL34 and 6L6s will work if you set the B+ low enough. The basic B+ is BTW set for 475.
I have used halls vvr in amps before and had acceptable results scaling the whole amp. I'm particular about power scaling, don't care for ppmv. The only limit to halls vvr is wattage ; don't exceed his spec or you'll be replacing the mosfet. I scaled a komet 60 clone and it sounded normal , all the way down. Tried to heat sink the mosfet but it burned up anyway.
I prefer to do the whole amp with VVR as well. I did switch to a different mosfet though.
Some of the newer ones are much more reliable than the old NTE 2973
I use these.
Mouser Link
As for heat sink,, simple mica wafer with a dab of white silicon goop. Bolt it to the chassis with the proper isolator hardware. NEVER had one of these fail.
Pushing a pair of KT77's at 475V and I can scale the amp to a whisper!
Some of the newer ones are much more reliable than the old NTE 2973
I use these.
Mouser Link
As for heat sink,, simple mica wafer with a dab of white silicon goop. Bolt it to the chassis with the proper isolator hardware. NEVER had one of these fail.
Pushing a pair of KT77's at 475V and I can scale the amp to a whisper!
Thanks for the recommendation for the STW15NK90Z MOSFET. I'll keep it in mind, though at $10 (CDN) apiece plus shipping, I think I work through my supply of 'old' MOSFETs first!
🙂
How 'unreliable' are the older MOSFETs, really? I mostly play with smaller amps, so probably not pushing them too hard (Here's hoping!).😀
The big thing I found was the fact that most guys never install the extra filter cap before the mosfet. Really common on retro-fits due to lack of chassis space.
IMO The extra cap is especially important to the life of the mosfet as spikes & ripple are pretty hard on those older end of life NTE's
I have fixed several for guys putting them in tweed 5E3's.
In every case, the standby switch on the B+ line had no snubber over the contacts. None had the extra filter before the VVR.
The website shows nice picture of the boards, and it is very well made & easy to install, but you do not know about the other additional parts needed until you actually buy it and read the instructions.
If you have the NTE 2973 its ok to use them. They have been out of production for some time now. Searching fleabay those run between $12 & $28US right now.
The STK from Mouser is $6.95US here.
I'm using an STW11NK100Z successfully in my Champ-type amp; they're well spec'd and cheap. It's working beautifully scaling the whole amp, but as Trout mentioned a 100u cap across the rectifier immediately before the VVR really helped reduce voltage spikes and more obviously: hum.
Still a bit pricey for me...($9 CDN+ shipping)
While we're talking components, what about the STP2NK90Z?
It's a lot cheaper (for me, less than $2 CDN + shipping) and rated at 900v and 2A.
Might work ok in lower power amps. Those have a pretty low current rating and a very low dissipation rating, be sure to use a really good heat sink.
I can pound on the one I mentioned about for hour after hour dimed in a 40+ watt amp and they don't even break a sweat.
- Status
- Not open for further replies.