All 7 pin tube amp—Video DemoClips Added 1/27/18

General discussion area for tube amps.

Moderators: pompeiisneaks, Colossal

Post Reply
tubeswell
Posts: 2337
Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: All 7 pin tube amp

Post by tubeswell »

brewdude wrote: Thu Sep 07, 2017 11:06 am I'm work in production--cellar lead. I usually figure I can't make any plans from middle of August to middle of November.
Whaddaya know? My brother is the same.
He who dies with the most tubes... wins
User avatar
RJ Guitars
Posts: 2662
Joined: Tue Nov 14, 2006 3:49 am
Location: Los Alamos, New Mexico
Contact:

Re: All 7 pin tube amp

Post by RJ Guitars »

Very nicely done. thanks for posting this up! I really like 7 pin tubes although there are really very few options for high gain 7 pin preamp triodes. I think most all-7-pin designs never caught on in a big way once high gain twin triodes were available in a 9 pin configuration. David Root put together a few amps using the 6AK5 pentode in the first gain stage and EL91's in the output stage... One great deception in all this is that a pair of 7 pin pentodes in the output stage can really put out some volume. Thanks again.
Good, Fast, or Cheap -- Pick two...

http://www.rjguitars.net
http://www.rjaudioresearch.com/
http://diyguitaramps.prophpbb.com/
User avatar
martin manning
Posts: 13079
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: All 7 pin tube amp

Post by martin manning »

If you really want 7-pin, 6AV6 is same spec as 12AX7 triode.
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

Thanks RJ.
Yeah, ~10 watts can be quite loud. Honestly, loud is not really something that I need. I like manageable volume levels and only really need to fill a small room... I'm no rock star. You're right though, there seem to be few high gain triodes available. Luckily, this amp doesn't require any. And, the 5725 seems to be a good inexpensive pentode. When the overtime checks start coming in, I plan on buying some more 6AQ5's to try and roll a few through.

I'm still intending to try a few tweaks optimize the VVR and PA bias.

I've also thought about messing with the pre amp pentode screen voltages to see what effect it might have. Perhaps, raise the screens on the first and lower on the second(?), or vise versa--Does any body have an opinion on such an arrangement?
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

Thanks Martin, I've had trouble acquiring 6AV6's and this build was all about the pentodes.
User avatar
martin manning
Posts: 13079
Joined: Sun Jul 06, 2008 12:43 am
Location: 39°06' N 84°30' W

Re: All 7 pin tube amp

Post by martin manning »

You could play with Blencowe's morph control, which transitions from pentode to triode.
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

I have an amp I built that has a paralleled 12ax7 input > TB tone stack > 6sj7 w/a VVR to control only the screen voltage > parallel 12au7 cathodyne > cathode biased 6v6's.

Changing the screen voltage has a pretty noticeable effect. At higher voltages it seems to be more glassy and lively and lower voltages more velvety and compressed.

If I were to make the two pentode stages dissimilar, which way do you think would be more desirable?
tubeswell
Posts: 2337
Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: All 7 pin tube amp

Post by tubeswell »

brewdude wrote: Thu Sep 07, 2017 11:51 pm If I were to make the two pentode stages dissimilar, which way do you think would be more desirable?
Turn one of them into a triode/pentode morph control - whichever one that doesn't have a significant AC load after it.
He who dies with the most tubes... wins
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

Plugged in this evening and cooler whether in No Cal had the AC voltage coming in at 121-122v and this pushed the plate to cathode voltage up to 251-253v--just over the stated limit--probably not an issue. So, I will leave the VVR's max limiting resistor as is.

I tried the cathode bias redistor at 150//330r=103r and dissipation was at 105%. Maybe, I should order a 120R resistor. Although, it's probably safer as is.

Also, it seems a bit tight. Do you think changing to 10uF filter caps in the pre and PI nodes (as in the original Gibson schematic) would be beneficial? Maybe it isn't so tight that it needs a change.

I did notice that it will squeal a bit when both gain and tone are dimed, dependent upon guitar proximity. I think I might add a grid stopper to the second gain stage which would have no resistance if both knobs are full on at "10".
User avatar
Kagliostro
Posts: 525
Joined: Wed Dec 30, 2009 12:09 am
Location: Italy

Re: All 7 pin tube amp

Post by Kagliostro »

Please can someone post a link to David Root amp with 6ak5 ?

Thanks

K
bluesbreaker62
Posts: 50
Joined: Fri Apr 06, 2007 2:57 pm
Location: Netherlands

Re: All 7 pin tube amp

Post by bluesbreaker62 »

Beautiful built right there!
Very interested in doing some 7-pin project myself.\

The EC91/6AQ4 is a high mu single triode, anyone use these before?

Edit: For a pentode pre, how about the 12AU6? Probably one of the few 7 pin tubes that have been used in guitar amp design (Kay, Silvertone)
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

Thanks bluesbreaker62.
I am quite pleased with the way it came together.
gtomax
Posts: 71
Joined: Mon Feb 27, 2017 6:52 pm

Re: All 7 pin tube amp

Post by gtomax »

Another cool use of chokes is the Gibson GA-90 where Seth Lover used a choke in the preamp to silence single coil hum. Worked like a charm too, the amp has a great warm clean sound. Scotty Moore used one for a while and it has this unique qty 6 of 8" speaker config too. Overall the amp is too early/clean to even get into blues territory - something the GA-40 doesn't have issue with, but it's a fun concept to incorporate into designs especially for P90's and strat/tele's.

https://elektrotanya.com/PREVIEWS/63463 ... .pdf_1.png

I'm also in norcal - nice to get a break from the oven we've been having!

Cool build!
brewdude
Posts: 651
Joined: Mon Mar 31, 2008 6:26 am
Location: Napa, CA

Re: All 7 pin tube amp

Post by brewdude »

Thanks gtomax,

Wow, that is a crazy circuit. Thanks for posting.
Is that inductor part of a tone circuit?
Looks like a cathodyne PI followed by a gain stage?

I like the cooler weather this week too.
User avatar
didit
Posts: 976
Joined: Thu Sep 25, 2008 3:37 pm
Location: Canada

Re: All 7 pin tube amp

Post by didit »

Kagliostro wrote: Fri Sep 08, 2017 6:33 am Please can someone post a link to David Root amp with 6ak5 ?
This might be it -- http://ampgarage.com/forum/viewtopic.php?t=28239
Post Reply