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What to do with extra triode?


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8/30/1998 8:54 PM
Pat F What to do with extra triode?
I recently finished a small practice amp housed as a head. I used a 6SJ7 pentode set up like an EF86 in an AC-10 for one channel and a 12AX7 using the same setup as the Spitfire in the other channel. These two channels go through 220K isolation resistors to a Champ style phase inverter and drive a single 6L6 (my OT came from an old Bogen PA using a single 6L6). Because I used cathodyne  
PI, I have an unused triode section. I was wondering if it would be detrimental to run both sections of the 12AX7 phase inverter tube in parallel for some extra gain. Would this cause problems because of impedance changes or other factors? I was also thinking of having a switch that disconnects the cathode of the second triode to take that section out of operation, making this parallel gain stage switchable.  
 
As it is, the amp sounds great. The main difference between the 6SJ7 and 12AX7 channels is the bass response on the 6SJ7 is much deeper and the 12AX7 is much brighter. Both channels offer plenty of gain, and the class A overdrive is ample. I used a variable ceramic cathode resistor for the output and set it to run about 17 watts disapation. The only draw back I've found is that the amp is a bit sensative to speaker vibration at high volume (I think because of the high gain of the 6SJ7) so the head has to be isolated from the speaker cabinet.  
 
Anyway, I'll entertain any thoughts about what to do with the extra gain stage. Thanks in advance.  
 
Pat
 
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8/31/1998 9:21 AM
anonymous
Might be possible to wire it up as an oscillator and use it for tremolo.
 
8/31/1998 2:00 PM
Doc

Pat:  
 
Did you use the resistor & voltage values in the AC-10 circuit, but apply them to a 6SJ7? It seems that many are interested in trying a pentode gain stage, rather than the most often used triode, but few successful circuits are published for them to try. There are a few older amp circuits publised using 6SJ7s, especially for a microphone input. Just wondering how you picked plate,screen & cathode resistors for yours.  
 
How did you wire up a cathodyne phase inverter to a single-ended output stage?  
 
Doc
 
8/31/1998 6:25 PM
Pat F
Doc - I meant to say I used the values from the "NORMAL INPUT" of the AC-15 circuit, which uses an EF86 pentode. The EF86 and 6SJ7 look similar on paper (except the pinouts of course) so I just used the values there. However I used a .047 coupling cap off the plate and 580pf on the bright switch (330 +250pf)instead of just the 250pf. After the bright switch I lifted the volume/tone section out of the AC-4. From there it goes through a 220K resistor directly to the grid of 1/2 a 12AX7 phase inverter. The PI and power amp tube (6L6) is set up exactly like that of the Champ AA764 model, except, as I said, I used a variable ceramic cathode resistor on the cathode that is set around 350 ohms. This transformer also has a 4,8 and 16 ohm tap.  
 
So, as you can see, not alot of "engineering" went into the values I chose. I checked the voltages on the 6SJ7 and none exceeded recommended maximums. The old Champ 5C1 used a 6SJ7 and it drives the power tube directly! Lots of gain there. I also found a 6SJ7 application on page 367 of my RCA tube manual (RC-19).  
 
In answer to your last question, how did I wire up a cathodyne PI to a single ended output stage? I didn't. When I posted my question, it was late, I was pooched from work, and I thought "single triode PI = cathodyne." Sorry about that. Rookie mistake. Hope I didn't mislead anyone.  
 
Cheers - Pat
 
9/1/1998 12:55 PM
Doc

OK, I see what you did. The PI you refer to in your circuit isn't a phase inverter, it's just a single voltage amplification stage. PI's are only required for a push-pull stage, where the signal and an inverted copy are fed to each of the two tubes' respective input grids. There isn't one on a Champ, or the single output tube amp you built. (BTW, your terminology of the single tube PI is correct..it's called cathodyne, or split-load. It's the type used on a Princeton Reverb, for instance.)  
 
I don't have access to an AC-15 schematic with the pentode preamp. For your 6SJ7, did you use resistors like 1.5k cathode, 220k plate and 1meg screen with a 0.1uf bypass cap?  
 
For the extra 12AX7 tube section, you could insert it between the your current "driver" and the 6L6. There will be way too much gain available, so you'd need to add a master volume control or tapped attenuator made with fixed resistors between the two sections of the 12AX7. Just an idea. By the way, there was a neat artice in Glass Audio magazine (vol.4, no.2, 1992) called "Remaking of a Champ", by Kirk Elliot, in which he set it up with a string of adjustable 12AX7 gain stages (3 separate volume controls) ahead of a single 6L6GC output into a 10" speaker, for a versatile studio amp.  
 
Good work, Pat.  
 
Doc
 
9/1/1998 3:03 PM
Steve M.

Hi Doc,  
 
>>>> By the way, there was a neat artice in Glass Audio magazine (vol.4, no.2,1992) called "Remaking of a Champ", by Kirk Elliot...<<<<  
 
I was just looking at that article last night. Have you actually built it? If so, how does it sound? I've been wanting to build this for a while, it looks pretty cool.  
 
Regards,  
 
Steve
 
9/1/1998 5:59 PM
Pat F
Thanks for correcting my error on the PI verses driver. I've always just used phase inverter and driver interchangably, but as you pointed out, there is in fact a difference.  
 
As for the values on the AC-15 EF86 pentode. The input goes through a 68K resistor, to grid 1. The cathode goes to ground via a 2.2K resistor bypassed by a 25 uf cap. Grid 3 is connected directly to the cathode. The plate load resistor is 220K. On the power side of the plate load resistor is an 8 uf filter cap (I used 16 uf because I had some) and from that junction is a 22K resistor going to power supply. Also from the 22k/220K junction is a 1 Meg resistor going to grid 2, and from the grid 2 + 1 Meg resistor junction a .1 uf cap goes to ground. Directly off the plate is a .02 coupling cap (I used a .047). That's it.  
Hope that all made sense. I probably don't have to tell you this, but since I drew it out on my schematic, the 6SJ7 pinouts are;  
Pin 1=shield, pin 2=heater, pin 3=grid 3, pin 4=grid 1, pin 5=cathode, pin 6=grid 2, pin 7=heater, pin 8=plate.  
 
As for cascading the extra triode, I too thought that would be far too much gain, hence my thoughts about running them parallel so there is some additional gain, but not nearly as much as in a series application. I guess I could just try it out and see what happens. The amp is on loan to a friend from work right now. Thanks for the reference to Glass Audio. I did at one time subscribe to it, and I have all my old issues buried somewhere in my back room. I'll check to see if I have that one.  
 
Thanks Doc - Pat
 

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