6SQ7,6SN7?

[gary sanders]

I just got two 6SQ7 and a 6SN7.How would these be in a pre amp?By looking at the data sheet the SQ has an amp. fact. of 100,would these be too microphonic?I heard the old octal triodes were in some cases.Anyway,they look cool and would look even better in a chassis!

[gary sanders]

pic.

[FYL]

I just got two 6SQ7 and a 6SN7.How would these be in a pre amp?By looking at the data sheet the SQ has an amp. fact. of 100,would these be too microphonic?

The 6SQ7 is more or less half a 12AX7 plus a diode in octal guise. Can be an excellent preamp tube if well used. Microphony can be a problem with some.

The 6SN7 is excellent as an output tube, a power tube driver or a phase inverter. Not enough gain for a front end.

[David Root]

Look up Gibson EH-150 Type 4 in Search. There are a few threads on it and I and others have built it. This amp was first sold in 1941 and was the last incarnation of the "Charlie Christian amp". It uses three 6SQ7, a 6N7 PI and a pair of 6L6G (NOT 6L6GC). You could substitute a 6SN7 PI, would probably be an improvement. 5U4G rectifier. 5U4GB will work for half the price.

Makes a "sleeper" type pre-tweed amp that is a pretty awesome 30W 1x12 combo especially with an old P12N which is how I did it. I used Tino Zottola's schematic which is good, although I went a bit lower in the PT voltage so as not to blow up my old Ken-Rad 6L6GAs. I actually overdid that and had to go to a 5R4GYB rectifier to get back to 400 or so Vp.

Sounds like a browned out old Marshall cranked in the mic channel.

[Phil_S]

6SN7 is about the same as 12AU7 in terms of electrical characteristics, but makes a far better output tube, probably due to the large plate size allowed by an octal bottle.

[tweedeluxe]

a 6SN7 cascode can easily reach gain levels beyond 100, plenty for an input stage. I've also used them with success as DC-coupled cathode followers in 5f6a/marshall type circuits, where the reduced gain doesn't matter much and can easily be compensated for.

[surfsup]

I've got a 6SN7 cascode input here, you can increase the gain with a higher Ra on the top triode:

http://ampgarage.com/forum/viewtopic.php?t=15968

Amp sounds nice, no hum. SE EL34...

[gary sanders]

I've got a 6SN7 cascode input here, you can increase the gain with a higher Ra on the top triode:

http://ampgarage.com/forum/viewtopic.php?t=15968

Amp sounds nice, no hum. SE EL34...

I was checking that out,got your schematic on the board

[gary sanders]

Look up Gibson EH-150 Type 4 in Search. There are a few threads on it and I and others have built it. This amp was first sold in 1941 and was the last incarnation of the "Charlie Christian amp". It uses three 6SQ7, a 6N7 PI and a pair of 6L6G (NOT 6L6GC). You could substitute a 6SN7 PI, would probably be an improvement. 5U4G rectifier. 5U4GB will work for half the price.

Makes a "sleeper" type pre-tweed amp that is a pretty awesome 30W 1x12 combo especially with an old P12N which is how I did it. I used Tino Zottola's schematic which is good, although I went a bit lower in the PT voltage so as not to blow up my old Ken-Rad 6L6GAs. I actually overdid that and had to go to a 5R4GYB rectifier to get back to 400 or so Vp.

Sounds like a browned out old Marshall cranked in the mic channel.

Man all I need is a PT and Ill have the parts for that.Will look for a sound clip on that.Is the OD a mod or did they really do an overdrive back then?Ive never heard of this amp

[Colossal]

Gary,

David and I were just discussing this amp offline. The voltages in the (original) amp are not as high as originally posted. I was mistaken and it was because I overlooked a very crucial piece of the puzzle. The original (Type III) EH-150 (around the 1940s) as well as other iterations of the amp used a Gibson T-40069 PT which was 408-0-408VAC on the secondary. The amp was touted as Class A1 push-pull but I thought there was no possible way it could be that as it should have 500V on the plates with that PT! Tino's version of it used a 365-0-365 PT but he kept the 200R cathode bias resistor. I have been pulling my hair out trying to make sense of how the EH150 wouldn't blow up those vintage 6L6Gs and GAs if it were running 500V on the plates! It turns out there is a 20k 20W bleeder resistor that drops about 237V off the main B+ giving a plate voltage of about 270V! This is certainly Class A1 territory and the 200R cathode bias resistor now makes sense (although the tubes still might be biased a bit hot).

As for overdrive, the Mic channel did indeed have overdrive and in spades. Have a listen to this:

http://prewaramps.org/media/H4.eh150.cs.lp.mp3

So a 365-400VAC secondary would do the trick nicely for an EH150 build. But the key is to add that 20k 20W to ground dropping resistor after the first Pi filter (before feeding B+ to the OT). Tino must've overlooked it too because he doesn't show it on his schematic. The build I did had considerably higher voltages but I made it work by taming the bias (800-1000R!) and it sounded killer. I'm gonna build a new one for myself with the right voltages!

Here is a photo of the amp I built for a guy.

[gary sanders]

Isnt that the same amp that Paul Reed Smith used for a demo on the Smithsonian Channels electric guitar documentary a few months ago?I looked up pics and it looks like it.

I was wondering about the voltages on it,the schematics I found on Hoffmans site didnt specify any.I may use EL84s in it since I have plenty of them laying around in the bin.

Notice in my pic I posted I have a 6K7.Would that work instead of the 6N7 for PI? It was just part of the lot of tubes I got.I just thought it was cool cause it was WW2 era but if it will work,Ill use it.I need a grid connector if anyone knows where to buy one(?)

[gary sanders]

Gary,

David and I were just discussing this amp offline. The voltages in the (original) amp are not as high as originally posted. I was mistaken and it was because I overlooked a very crucial piece of the puzzle. The original (Type III) EH-150 (around the 1940s) as well as other iterations of the amp used a Gibson T-40069 PT which was 408-0-408VAC on the secondary. The amp was touted as Class A1 push-pull but I thought there was no possible way it could be that as it should have 500V on the plates with that PT! Tino's version of it used a 365-0-365 PT but he kept the 200R cathode bias resistor. I have been pulling my hair out trying to make sense of how the EH150 wouldn't blow up those vintage 6L6Gs and GAs if it were running 500V on the plates! It turns out there is a 20k 20W bleeder resistor that drops about 237V off the main B+ giving a plate voltage of about 270V! This is certainly Class A1 territory and the 200R cathode bias resistor now makes sense (although the tubes still might be biased a bit hot).

As for overdrive, the Mic channel did indeed have overdrive and in spades. Have a listen to this:

http://prewaramps.org/media/H4.eh150.cs.lp.mp3

So a 365-400VAC secondary would do the trick nicely for an EH150 build. But the key is to add that 20k 20W to ground dropping resistor after the first Pi filter (before feeding B+ to the OT). Tino must've overlooked it too because he doesn't show it on his schematic. The build I did had considerably higher voltages but I made it work by taming the bias (800-1000R!) and it sounded killer. I'm gonna build a new one for myself with the right voltages!

Here is a photo of the amp I built for a guy.

That sounds really good and looks cool too.I like seeing how much stuff I can cram into a chassis.This could very well be the pre amp I use in my Conn organ amp with PP 7868s.

Where did you get the face plate on the build in the pic?

[Colossal]

Where did you get the face plate on the build in the pic?

Gary,

I drew it up and then got it laser engraved through BNP Lasers.

Notice in my pic I posted I have a 6K7.Would that work instead of the 6N7 for PI? It was just part of the lot of tubes I got.I just thought it was cool cause it was WW2 era but if it will work,Ill use it.I need a grid connector if anyone knows where to buy one(?)

No, a 6K7 would not work in place of the 6N7 unfortunately. A 6N7 is dual triode but with an internally shared cathode. You can pick up 6N7s pretty cheap however.

[gary sanders]

Yep,found the datsheet on that,its a pentode.Will use the 6SN7.

what version is the hand drawn schematic posted above?I found several versions of the amp

[overtone]

...It turns out there is a 20k 20W bleeder resistor that drops about 237V off the main B+ giving a plate voltage of about 270V! ...

Damn that clip sounds good.
Last week I was going through Robin & Lipman 1947 Practical Amplifier Diagrams and I kept on noticing exactly those kind of 20W bleeder arrangements. Thanks, I had not grasped why they chose to do that.