I picked up a couple Type 42 tubes from a lot this weekend. They look so classic that I want to build a single ended very vintage hi-fi (yes, I some may think this is a mis-use of the word) amp around them. I know I won't get much power and there's lots of talk about distortion, but I think I could work around it and be happy with the results.
Because part of the attraction is the "look" I want to stay with a 2 digit 6 pin driver. What is everyone's thoughts for a driver. To my uninformed search, it seems a type 76 is what I should target.
Thoughts?
If anyone has a schematic that would be nice also.
Thanks in advance.
Because part of the attraction is the "look" I want to stay with a 2 digit 6 pin driver. What is everyone's thoughts for a driver. To my uninformed search, it seems a type 76 is what I should target.
Thoughts?
If anyone has a schematic that would be nice also.
Thanks in advance.
Last edited:
Thanks everyone!!
To summarize my goal in one sentence; Think of a classic car, with upgraded engine, brakes and tires. Looks awesome and performs better than original.
So to narrow the field a little, here is what is important.
- Not watts
- I said 6-pin because I want a "period" piece. The "look". But I am not stuck on 6 pin only. In fact a top cap would be kinda neat, but don't want to limit my choices to that either.
- The 42's come in coke bottle and globes. I have the globes (I think they look even more vintage, and that is my goal). So readily available, not over priced, good sounding globes would be nice. Otherwise, only coke bottles. In my opinion, only globes or coke bottles will get the look I want.
- 6.3 filiments because I don't want the PS to get too complicated. I'm avoiding complicated; single ended etc....
- While I list this last, I do want it to sound good. I will listen to it - maybe alot if i decide I want it in my at-home office. (I work from home alot)
Given the distortion of the 42 tube in pentode, I would probably try triode and maybe UL. The likely speakers (A25's in my office) triode may not be enough, so UL is attractive, but I haven't looked at it enough. I have a home grown SE EL84 in UL in there now. Speaks prolly won't change unless I stumble over some nice Hereseys for a good price (just missed a set for $350 on Monday!).
To summarize my goal in one sentence; Think of a classic car, with upgraded engine, brakes and tires. Looks awesome and performs better than original.
So to narrow the field a little, here is what is important.
- Not watts
- I said 6-pin because I want a "period" piece. The "look". But I am not stuck on 6 pin only. In fact a top cap would be kinda neat, but don't want to limit my choices to that either.
- The 42's come in coke bottle and globes. I have the globes (I think they look even more vintage, and that is my goal). So readily available, not over priced, good sounding globes would be nice. Otherwise, only coke bottles. In my opinion, only globes or coke bottles will get the look I want.
- 6.3 filiments because I don't want the PS to get too complicated. I'm avoiding complicated; single ended etc....
- While I list this last, I do want it to sound good. I will listen to it - maybe alot if i decide I want it in my at-home office. (I work from home alot)
Given the distortion of the 42 tube in pentode, I would probably try triode and maybe UL. The likely speakers (A25's in my office) triode may not be enough, so UL is attractive, but I haven't looked at it enough. I have a home grown SE EL84 in UL in there now. Speaks prolly won't change unless I stumble over some nice Hereseys for a good price (just missed a set for $350 on Monday!).
First, Thanks!! for everyone's thoughts. Been spending alot of time surfing on this idea. Don't tell my boss...
I have no problem with a tube with a plate cap. In fact it might add to the retro look.
I will try to mock up the look I'm trying to get tonight and send pics so everyone will get the idea. Much of this little project is about "the look". Just don't want to botch up the sound under the hood.
P.S> The radio also came with 2x 36 tubes. Not same brand and I've not tested them yet. Nor looked at the data sheets. They don't look like tubes for a driver stage, but I've heard of people using power tubes as drivers - not exactly something I have tried to understand. But I have the tubes already.
I have no problem with a tube with a plate cap. In fact it might add to the retro look.
I will try to mock up the look I'm trying to get tonight and send pics so everyone will get the idea. Much of this little project is about "the look". Just don't want to botch up the sound under the hood.
P.S> The radio also came with 2x 36 tubes. Not same brand and I've not tested them yet. Nor looked at the data sheets. They don't look like tubes for a driver stage, but I've heard of people using power tubes as drivers - not exactly something I have tried to understand. But I have the tubes already.
So just wire 36 up like any driver tube? I'm planning cathode bias on the driver.
How about 250 on the plates of both driver and power tube. Triode strap the 36 and enough plate resistance to keep below 3.2ma on the plate and cathode resistor enough to idle at -3 on G1. I don't have plate curves or a good data sheet, so will need to do better math later. Sound about right?
How about 250 on the plates of both driver and power tube. Triode strap the 36 and enough plate resistance to keep below 3.2ma on the plate and cathode resistor enough to idle at -3 on G1. I don't have plate curves or a good data sheet, so will need to do better math later. Sound about right?
Being a tetrode, the 36 has ugly plate curves:
http://tubedata.tigahost.com/tubedata/sheets/029/3/36.pdf
As you already have the tubes, it won't cost much to try them.
If you triode them, watch the low screen rating...
http://tubedata.tigahost.com/tubedata/sheets/029/3/36.pdf
As you already have the tubes, it won't cost much to try them.
If you triode them, watch the low screen rating...
6V equivalent is 6A6 or octal 6N7G. Dual triode with common cathode. If you bypass the cathode you can use half for each channel.
I use 36 with operating conditions from datasheet:
Upalte=180v, Uscreen=90v, Ugrid=2v
It was a good sound
I have often wondered about this. I have lots of 6N7G and 6J6, but thought that this sort of use wasn't possible. Can you expand on this, please?
75, 76? 75s has a high mu of 100 a lot like a 12AX7, but they have two diodes which I just ground off & ignore. You will want to shield that 76! I have used the 42 myself, in lap steel, and harp amps, with either a 75 or a 76 tube for a driver SE, or 3 or 4 of them for push pull amps. They look cool, & done right they can sound quite nice too! The 76 is a bit like a 6C6 which was also used in OLD lap steel amps. Those are always shielded in the old Goat type shields. Thick copper foil with a ground wire will work too. Carefully cut or fold the foil to contour to the tube, & solder on a ground wire. Mark where the foil meets up take it off & solder those points until it fits fairly well, then wrap the tube & tack the foil together with more solder. I have seen the foil method more in English radios, & some test gear, but it is done. You can get those Goat style split shields on eBay too.
So I’ve been away from my post for a couple days. Work keeps getting in the way! Thanks all for the ideas!!
Anyway, I think I’m gonna try the 36’s first and if that doesn’t work, I will go with 6A6 or octal 6N7G.
So at this point its gonna be a 42 SE (not sure about Triode or UL or Pentode – will prolly try all 3. If UL works, that is my plan “A”) with 36 triode strapped drivers.
I want to run the 36 triode strapped and with local feedback. So given the screen’s 90v limitation, how should I implement as a triode? A dropping resistor from Plate to Screen or a dividing network (2 resistors in series from plate to ground and the Screen coming off between the 2 resistors)?
As someone pointed out the 36 driver curves have a lot of distortion. Does local feedback make sense? Is it needed to make the driver more linear. What did you do oleg? BTW this is a hi-fi amp, not an instrument amp.
Thanks again and looking forward to what discussion this prompts.
Anyway, I think I’m gonna try the 36’s first and if that doesn’t work, I will go with 6A6 or octal 6N7G.
So at this point its gonna be a 42 SE (not sure about Triode or UL or Pentode – will prolly try all 3. If UL works, that is my plan “A”) with 36 triode strapped drivers.
I want to run the 36 triode strapped and with local feedback. So given the screen’s 90v limitation, how should I implement as a triode? A dropping resistor from Plate to Screen or a dividing network (2 resistors in series from plate to ground and the Screen coming off between the 2 resistors)?
As someone pointed out the 36 driver curves have a lot of distortion. Does local feedback make sense? Is it needed to make the driver more linear. What did you do oleg? BTW this is a hi-fi amp, not an instrument amp.
Thanks again and looking forward to what discussion this prompts.
I used type 42 tubes. I liked the sound of both pentode and a triode. You can make the switch and quickly choose the mode. What output transformers will you use?
In my amplifier 36 tube is in pentode mode. I use a two resistors in series for screen limitation. Also potentiometer can be used for the selection of these resistances. I did not do any feedbacks, but I think you should necessarily try it.
In my amplifier 36 tube is in pentode mode. I use a two resistors in series for screen limitation. Also potentiometer can be used for the selection of these resistances. I did not do any feedbacks, but I think you should necessarily try it.
Thanks Oleg. Couple questions (and answers...)
When you wired your 36 screen supply, did the 2 resistors in series go from plate to ground (center to screen - right) or B+ to ground (again, center to screen). The first would seem to be more like a Triode and the second would be Pentode. Yes, I plan to use a pot to tune the screen voltage.
The only OPTs I have are from a el84 SE I rebuilt. The are small, but sounded pretty good. I never tested them, but expect they are in the 5K range, and the 42 says 7k with a 250v B+. I have a regulated supply so it would be easy to try the 42 @ say 200 with the 5k just to see how it goes. But I expect I will be buying some OPTs for this project.
If anyone has any better ideas for how to use the 5k SE OPTs with the 42s that would be good. However, I am planning to try to get good sound out of this amp, so if I use the 5k ones it would be part of experimenting, unless by some chance they actually sound good.
When you wired your 36 screen supply, did the 2 resistors in series go from plate to ground (center to screen - right) or B+ to ground (again, center to screen). The first would seem to be more like a Triode and the second would be Pentode. Yes, I plan to use a pot to tune the screen voltage.
The only OPTs I have are from a el84 SE I rebuilt. The are small, but sounded pretty good. I never tested them, but expect they are in the 5K range, and the 42 says 7k with a 250v B+. I have a regulated supply so it would be easy to try the 42 @ say 200 with the 5k just to see how it goes. But I expect I will be buying some OPTs for this project.
If anyone has any better ideas for how to use the 5k SE OPTs with the 42s that would be good. However, I am planning to try to get good sound out of this amp, so if I use the 5k ones it would be part of experimenting, unless by some chance they actually sound good.