Swap out different Preamp Tube Type ?

[HaroldBrooks]

Is going from one preamp tube to another just a matter of making sure the tube socket pins are connected to the correct wires, as in #3 = plate, #2 & #7 = heater, etc... Is that all that needs to be take into account, provided the voltages are more or less correct, and provided I use the proper grid, bias, and Plate resistors ?

I am looking into swapping to another V1 tube with a higher amplification factor than the original, in hopes the new V1 tube can drive everything downstream a bit more, rather then the amp distorting at V1 heavily first. Swap is from a 6J5 to a 6SQ7.

Just want to make sure I am not oversimplifying the whole thing.

Thanks for your help !

[The Dude]

The pinout of those two tubes is quite different, or are you saying you plan to rewire the socket?

[Chuck H]

Looking at the data sheets, as far as I can tell the 6sq7 is not compatible with a socket wired for a 6j5. So I guess I would say that just plugging a 6sq7 into a socket wired for a 6j5 IS oversimplifying it.?. As would be the stock load for the 6j5 compared to that for a 6sq7. Were you planning on rewiring the sockets and changing some component values with the swap?

[The Dude]

I'll also add that it should be difficult to clip the very first gain stage by itself. Have you scoped the amp to verify that is where the distortion is coming from? If so, I suspect circuit problems other than the tube or tube type.

Maybe a schematic so we can see what you're working with?

[Dave H]

I am looking into swapping to another V1 tube with a higher amplification factor than the original, in hopes the new V1 tube can drive everything downstream a bit more, rather then the amp distorting at V1 heavily first.

That sounds backwards to me. If it's heavily clipping at V1 it needs a tube with less amplification not more. A tube with more amplification will make it clip even harder. Also if it's already clipping at V1 it won't be able to drive the following stages any harder.

[Chuck H]

That sounds backwards to me. If it's heavily clipping at V1 it needs a tube with less amplification not more. A tube with more amplification will make it clip even harder. Also if it's already clipping at V1 it won't be able to drive the following stages any harder.

I got the impression that he's driving the input with a booster, clipping V1. Then, since V1 only has an AF around 20 he's not getting satisfaction in the later stages.?. But I can't be sure.

[HaroldBrooks]

That's correct Chuck H. Sorry, I didn't describe it well enough, yes I will rewire the socket to accommodate the 'New' and different 6SQ7 pin outs. Just wondering if I am overlooking anything else besides the difference in cathode, grid leak, and plate resistors.

Yes, I am driving the tube with a boost up-front, but that sounds like crap alone. When you don't use a boost, the amps later stages don't seem to be running into distortion, perhaps because of the V1 lower gain.

I've never designed any amp in my life, so I don't know what is essential, and what is not regarding a match between stages.

I don't have a scope, but I sure should get one and learn how to use it at this point !!!

Am I missing anything else ?

[The Dude]

I'm guessing with no schematic, of course. But, wouldn't it be easier to mod the circuit slightly rather than reconfigure for a new tube? Plate resistor, cathode cap, etc. There ought to be plenty of easier ways to get more gain out of the circuit, if that's what you want.

[HaroldBrooks]

I'm guessing with no schematic, of course. But, wouldn't it be easier to mod the circuit slightly rather than reconfigure for a new tube? Plate resistor, cathode cap, etc. There ought to be plenty of easier ways to get more gain out of the circuit, if that's what you want.

Just found the schematic (It's seems very close at first glance) online. I will take your advice and first measure everything over the weekend, to see if it's in spec or not. As usual for me, my enthusiasm may lead me to putting the cart before the horse and jump to something more radical like an altogether different tube swap. Instead now I will play around with what I have and see how far I can get, before making a more radical change. I still haven't checked all the resistors, voltages, etc. and that should come first to see what I yield with the existing design and parts.

Thanks for sobering me up at this point ! I will let everyone know how things progress with this. I reached a type of "Wall" with my current crop of guitar amps, that is I've modified every last one, some more, some less, and they are all working great... Leaving me now with nothing to do. This new project will create just the trouble I need to stay active with tube amps !

[Chuck H]

Just found the schematic (It's seems very close at first glance) online.

Could you post it or link it please? Or at least tell us what the amp is so WE can try to look it up?

[Chuck H]

In the meantime... I found this gadget. The description is a little confusing, I think there's a language barrier. But this adapter seems to allow you to plug in a parallel 12au7 tube, or perhaps link one of it's triodes, to replace the 6j5 triode in the circuit. No socket replacement or rewiring. Since a 12au7 pinout is the same as other 12**7 tubes you could use this adaptor to plug in a 12ay7 or a even a 12ax7 provided the plate and cathode resistors in the circuit are appropriate. This would provide a considerable increase in gain. But I'd like to see a schematic before endorsing this maneuver.

https://www.ebay.com/itm/1Piece-Gold...-/202437648734

[HaroldBrooks]

In the meantime... I found this gadget. The description is a little confusing, I think there's a language barrier. But this adapter seems to allow you to plug in a parallel 12au7 tube, or perhaps link one of it's triodes, to replace the 6j5 triode in the circuit. No socket replacement or rewiring. Since a 12au7 pinout is the same as other 12**7 tubes you could use this adaptor to plug in a 12ay7 or a even a 12ax7 provided the plate and cathode resistors in the circuit are appropriate. This would provide a considerable increase in gain. But I'd like to see a schematic before endorsing this maneuver.

https://www.ebay.com/itm/1Piece-Gold...-/202437648734

Been busy with family stuff, sorry for the delay.

Here's the schematic on page 182 :

http://www.nucow.com/SkizsPdfOnly/Su...lumes/1957.pdf

It's an old Zenith Hi-Fi amp, but looking at the tubes, I realized it's close enough for a guitar amp, after a couple of minor tweaks. Should have said this from the start, but I was planning a trip and typing too quickly, so I left things out in the original posts.

It appears I jumped the gun on this one (again !), as I found a few issues that were causing the problem, starting with the way I was hooking up the speaker, and ending with the bad wax coupling caps to the 6V6's. I've straightened those things out, and the amp is now a screamer, even with the original 6J5 still in place.

I would still like to get an adapter just for the heck of it, and see what it sounds like with a higher amplification factor 12AX7 up front in V1, or perhaps I will wire up an octal 6SQ7 just to see if that sounds any better. All in all this "new" 1957 amp makes a great guitar amp. It's already got a pretty good sounding presence circuit built in. It was originally a Hi-Fi Zenith amp, and I might play with the values of the cap to see if I can make even better. This amp is a pleasant surprise for me !

Once again, Thanks for your patience and help Chuck !

And thanks to everyone else as well ! You guys are all super !

[Chuck H]

My only caveat for the adapter would be to test the pinout to see if it isolates one triode or if it puts them in parallel for the dual triode substitute. Because the description doesn't really say. My guess is that it isolates one triode.

[HaroldBrooks]

My only caveat for the adapter would be to test the pinout to see if it isolates one triode or if it puts them in parallel for the dual triode substitute. Because the description doesn't really say. My guess is that it isolates one triode.

Probably the single triode, I would guess as well. This "amp" is going to be a lot of fun for me, as it is close to a guitar amp by design, but I just received it as a chassis, so I can dig in at length without worrying about stripping wood screws that results from taking a real amp apart too many times.

I might see if I can change some of the parameters of the input selector switch, as it was designed for the Records / interfaces of the day, but I'm not joking when I tell you the amp sounds quite good, as it sits. The Wax coupling caps I replaces were 600V .1uf Sprague units, and I put poly 47nf 630v ones in place. I'm going to see if I can bump up the gain a bit with cathode bypass caps, and check the bias of everything, along with cleaning further.

Thanks for the help, and I will buy that adapter today.

[HaroldBrooks]

My only caveat for the adapter would be to test the pinout to see if it isolates one triode or if it puts them in parallel for the dual triode substitute. Because the description doesn't really say. My guess is that it isolates one triode.

Made some changes to the amp to increase gain and change the distortion / tone.

First thing I did was to knock back C9 (see negative feedback circuit) from .047uf down to .005uf, while eliminating the corresponding 39k resistor all together. This added some needed gain and changed the tone of the presence control. I tried several resistor capacitor combinations, and this one (just using a smaller cap) seemed to create the best presence tone (a bit hairy on the highs) while not adding some, but not too much gain, so as to destroy the already smooth distortion all together.

The Bass and Treble controls behave rather oddly when the amp being pushed hard, as they seem to add too much, or subtract too much. Not sure what's going on here, but I dropped the .047uf cap to a .022uf for the treble circuit, and it helped a little. When you back off the treble entirely in the stock configuration, it seems to boost the super low frequencies and cause a nasty type of blocking distortion. Same with turning the Bass up fully. Not too worry though, if you adjust both controls appropriate to the high level gain (as in death metal level gain), the amp does behave itself, and the distortion is quite smooth, so it seems you just have to be aware of this.

I also backed off on the size of the C11 and C12 Coupling caps to the 6V6s from .1uf to .047uf. They were leaky, so I took the opportunity to make them smaller as well. I also found a couple of plate resistors (the ones for the PI tube ) that were out of spec in ohms, so I replaced those and the voltages match the schematic for the two 12ax7 plates now. The plates on the 6v6s are 355vdc and the grids are 315vdc, so that seems good as well. No red plating, but the tubes are biased close to maximum one being 13.7 watts and the other 12.6 watts dissipation, so I am going to leave it as is.

At this point is sounds so good, I probably will just stick with the 6J5 preamp tube, but when the adapter arrives I will try the 12ax7 just to see how it works.

Any other suggestions are welcomed ! I am experimenting, so I don't mind making changes and then reversing them, depending on what transpires. Nice to have an inexpensive test amp like this ! It's a cheap but usable toy for me, something I only have time into, but it's similar to some of my other amps.\

I am intrigued most by the presence circuit on this little amp, and the way it screams when driven beyond all good sense, using a lot of boost up front.