Tube selection in Tweed Overdrive Special

[daveyajd]

Hello all,
 Well after a very busy summer and many hours spent playing my TOS I can say I am extremely happy with this amp. I am finally getting it put into something resembling a cabinet. My ultimate goal is to get this built into a nice combo with a single 12" Jensen but for now I have done a moderately well build head cabinet that I still need to cover with tolex.


So now I am looking at rolling some new tube combinations for fun. I have been using JJ 6L6s and plan on getting some 5881s to try out. Also going to try a 12AY7 in the PI and maybe V1. I am still using the SS rect. but I do have a GZ34 and a 5U4GB laying around. Any thoughts on different rectifiers? I also remember reading one of the early threads on this amp where someone mentions using EL34s in this amp. What changes would be necessary to use those tubes? Could I then switch between different PT types? Thanks for all the help along the way.


Aaron

[daveyajd]

So I ordered a set of Tung Sol 5881s. I built this amp pretty much exactly to the schematic. Slightly different PT and OT I think. I am using a SS rect. plug and have voltages of 464v on my power tube plates. Do I need to adjust this voltage with a tube rectifier to bring those voltages down? Seems to me these tubes are spec'd for something closer to 400v.


Aaron

[tubenit]

I would rather have a lower voltage on Tung-Sol 5881's,  so it might be worth trying a rectifier.  I usually use GZ34 or 5V4's (which I get from Doug).  I like both of those.   

My guess is that you have about 330-0-330 PT?   So 330 x 1.4 (solid state) = around 460v or so.

GZ34 should put you around 430v ??         5V4 or 5U4gt should give you maybe 396v ??

Also, let your ears be the judge of what you like best.

I would not use a 12AY7 in the LTPI.  I'd use a 12AT7 or 5751 but not 12AY7.

I think 12AY7 could sound excellent in V1 ............ or a 5751 in V1 also.  I don't hear 12AT7 as particularly musical in a V1.

With respect, Tubenit

[daveyajd]

Thank you. My PT is a Hammond 290DX which according to its specs is a 325-0-325 so that should work. I put in a GZ34 and played it for a minute. Sounded great and I will measure the voltages later when I can pull the chassis out. So when I receive my 5881s do I risk having something happen if my voltage is still in the 430v range? Also, I will try your suggestion of either a 12ay7 or 5751 in V1.


Anybody have experience using EL34s with this amp? Opinions? I have found info for converting 6L6 amps to use EL34. Changing the 470 ohm resistors between pin 4 and 6 to something like 1000 ohm, floating the wires and resistors on pin 1 and connecting pins 1 and 8. Does that sound correct? If so that could be done fairly easily to allow for more tube options. I am attaching a picture of the amp as it sits right now as a head. I really do want to put it all together in a combo but this at least allows me to present it as complete.


Aaron

[tubenit]

There are some important things to know about the reissue Tungsol 5881 tube before you decide to use it. The Tungsol 5881 is a 23 watt tube and is rated for a maximum plate voltage of 400Vdc. The many amplifiers that have higher plate voltage but use a tube rectifier can use the Tungsol 5881 tubes without issue. A prime example is the '59 Bassman LTD. However it is also critical to rebias the amp heeding the 5881's 23W rating. This will provide a vintage tone and less clean headroom. Leaving the bias set to a 6L6GC setting and/or using the Tungsol 5881 in a high voltage amp with a solid-state rectifier, will likely cause problems and are not covered by warranty. For these cases consider the Tung-sol 6L6GC-STR tubes instead. They are rated for 30 watts and 500V so they will hold up better. Their tone is quite similar to the 5881 too.

If you've never tried one of Hoffman's 5V4 rectifier tubes,  I can recommend them.  I prefer it more then the 5U4Gt which I also own.

THANKS for posting the photo of your amp!  That looks really great. Nice job on the face plate.

with respect, Tubenit

[ampgeek]

What brand 5881 did you end up with?

Regardless, the JJ 5881 offering is good to 500 VDC on the plates when used as a pentode.  Somewhat lower (450 maybe?) in triode mode.

The Sovtek 5881 WXT is ok at even higher plate voltages.

So....you will be OK at ~430 VDC on the plates with those two new production tubes.

Rock on,
Dave O.

[daveyajd]

I ended up ordering the Tung Sol 5881s. Reissues not NOS.

[daveyajd]

Ok, so I got a chance to swap some rectifiers and take some measurements. The GZ34 drops the voltage to 440. My 5U4GB drops it to 413. I have an Tung Sol 5Y3GT, do I dare put that in? That will bring the voltage well below 400 I'm guessing but will I suffer any negative effects? Is 413 close enough to the 400 rating? I will get my 5881s tomorrow. I'm excited to check them out. If I like the sound I may order some JJs that will allow for more voltage.


On a side note I discovered something in the way my amp controls work that I had failed to notice before. This is probably due to the fact that I had not played with the "clean" channel controls while playing on the "overdrive". The gain and tone on the clean side also are also affecting the sound when I am playing on overdrive.


Aaron

[tubenit]

The original TOS had a 430 ohm 10w resistor.  Put in a 270 ohm to 330 ohm range 10w resistor and then install the 5881's and measure plate voltage and current draw.

I think you will find the voltage to decrease and the current to increase?

I think the "actual" plate voltage involves subtracting the voltage from the cathode?  Maybe sluckey can weigh in on that?

I would NOT use the 5Y3 rectifier.  There were some Silvertone amps that used a 5Y3 with 6L6's but I would not do that.

With respect, Tubenit

[daveyajd]

I built mine with a 360 ohm resistor because a 430 was not available anywhere. I will look and see if I happen to have a 300 ohm at all. I measured my plate current using my bias probe and got 36.6 mA on one tube and 42.3 mA on the other. This is with the 5U4GB and 412v on the plates. What other measurements would I need to find "actual" plate voltage?


Aaron

[sluckey]

What other measurements would I need to find "actual" plate voltage?

Measure the cathode (pin 8) voltage. Subtract the measured cathode voltage from the measured plate voltage to find the true plate (to cathode) voltage.

[daveyajd]

Pin 8 measures 36v. With the 5U4 that puts me well below 400v and with the GZ34 it puts me right at about 406v. I did find that I have a 300 ohm resistor but I may wait to do that change since I am getting a satisfactory plate voltage anyway.


Aaron

[tubenit]

Sounds like you may be OK already?

I'd give the 300ohm/10w a try and measure current after putting the 5881's in.  IF you try that, repost the voltages and current please.

With respect, Tubenit

[jojokeo]

If there's no red-plating then everything should be fine. But for the sake of due diligence it's always good to know where things stand. 6L6/5881 at 405Vdc is between 29.6mA-35.2mA. At 450 it should be 26.6mA-31.7mA. It's not advisable to use under-rated 5y3 with 6L6/5881s.

[daveyajd]

Got my 5881s today. Put them in and gave them a brief test run. They sound good. I don't know that I have a very accurate ear that can tell the difference between them and a JJ 6L6 but either way it is nice to have a new set of matched tubes. I put the meter on to take measurements and measured 439 v in the plate and 39 on the cathode. The tubes look fine and there is no red plating so I feel they are in a safe place. I'm going to put my bias probe in to check current this evening.


I had a fleeting thought last night as I drifted off that I may have a 12AY7 in my small box of tubes I collected from some Bogen amps I kicked around for awhile. When I went to look today there was no 12AY7. Mostly oddly marked tubes that I don't recognize but I did find these. 2 GE 5V4GA rectifiers! After I play with these new 5881s for a bit I may put one of these in to see how they work.


Aaron

[daveyajd]

Everything has been working great on the amp with the new tubes.....until today. I have been tweaking a little by jumpering a parallel 25uf cap with the 2.2 uf cap on the 5879 cathode. Everything was sounding great. That cap really does add something nice to the OD. I would go back and forth playing with and without. I came back a while later to play some more, hooked the cap up(correct orientation), flipped the switch on and the fuse popped immediately. I unhooked the cap, replaced the fuse, flipped the switch and played the clean channel for a sec. Switched to OD and when I dug in the fuse popped again. Now I'm out of fuses and will have to find a few replacements. Where should I be looking for this issue?

Aaron

[jojokeo]

Check the rectifier tube and anything else along your B+ and/or power tranny primaries and secondaries to be off or out of whack. Unlikely it's related to your cathode bypassing you're playing with - just coincidental.

[tubenit]

Jojokeo has given you great advice! 

With respect, Tubenit

[daveyajd]

Thank you. Would my new 5881s be a possible cause here? There has been no red plating or anything but maybe because my plate voltage is just at or above the recommended 400v when I push the amp on OD they can't handle it. I have to get some fuses and then I can take some measurements.

Aaron

[tubenit]

Put tubes in that are rated for well more then 400v like JJ 6V6 or JJ 6L6 and try those AFTER ruling out rectifier and B+ rail wiring problems.  Be sure to look for traces of  "arcing" on the power tube sockets.  IF you replaced the cathode resistor ......... make sure that the soldering joints are right with that and that the cathodes are grounded well.     Look for loose wires touching the wrong pins etc ........

If you've made Hoffman's "bias checker" or have 1 ohm resistors set up to check .............. check the ma for each of the 5881's  IF you suspect they may be the problem.  Maybe they are not close to a matched set?

PLEASE post what you find out.    With respect, Tubenit

[daveyajd]

I will do another inspection looking for shorting points. I have not changed the cathode resistor yet. In fact I haven't done any real tweaking other than the jumper on the OD cathode. When I checked the bias of the 5881 upon initial install they measured 51 and 50 mA. I will check them again. I was able to pick up a few fuses at a local surplus store. They didn't have any 2A but they had 3A. Will those work safely? Even if just to power to test voltages. I'm not sure if they are slow blo either but they look similar to the slo blo fuses I have had.

Aaron

[tubenit]

Tweed Pro 5E5 which is cathode biased (dual) 6L6 uses a 3amp fuse.

Tweed Pro 5C5 which is cathode biased (dual) 6L6 uses a 2 amp fuse.

Super Reverb which is fixed biased (dual) 6L6 uses a 3 amp fuse.

6G5 which is fixed biased (dual) 6L6 uses a 3amp fuse.

Answer to your question about using a 3A fuse?  I don't know. 

With respect, Tubenit

[daveyajd]

 
That should have ended: Finding out if your amp uses a 3 amp fuse, priceless.

Your schematic called for 2A so I will have to order those eventually but I think this will let me power up to get a few voltages. Thanks.

Aaron

[tubenit]

OK,  my current schematic for my Tweed BluezMeister using cathode biased JJ 6L6's (or 6V6's sometimes) shows 3A.

My current schematic for my D'Mars ODS using cathode biased Tung Sol 5881's shows a 3A.

I am thinking I have somewhere between/around   360v using a 5V4GT..... to 390V using a GZ32 ..........to 420v using a solid state rectifier.

with respect, Tubenit

[daveyajd]

Ok got home and inspected all connections for possible shorts with the wiring. Everything looks good. Removed the 5881s and put my 6L6s back in. Put a new 3A fuse in flipped the switch...POP! I removed the JJ GZ34 and put my SS back in. New fuse, flip the switch.....works perfectly. I took some voltages and worked my way back through my rect tubes. Here is what I found.


SS - Plate 460v Cathode 37v = 423v actual
5V4GA - Plate 428v Cathode 34 = 394v actual
5U4GB - Plate 409v Cathode 33v = 376v actual
GZ34 - shorting sound....POP


I guess my GZ34 gave up the ghost. Bummer.


I put the 5881s back in and the 5V4GA and everything seems to be working great! Problem solved. 390v actual plate.
I did go through a few fuses but the surplus store I have here in town has a gross of these 3A fuses and I can buy them for a quarter each. I am going to scoop up a few more just to have for future builds at that price. Thanks for the help everyone.


Aaron

[tubenit]

Glad you got the issue resolved and I appreciate you sharing the success!

I do hope at some point that you will try a smaller value cathode resistor like 270R to 330R range.  I think you might like the tone of it with those tubes pushed a little more.   When we originally used 430R on the design, it was to try and get a "cleaner" power amp tone and have less oomph there.

With respect, Tubenit

[jojokeo]

SS - Plate 460v Cathode 37v = 423v actual
5V4GA - Plate 428v Cathode 34 = 394v actual
5U4GB - Plate 409v Cathode 33v = 376v actual
GZ34 - shorting sound....POP


I guess my GZ34 gave up the ghost. Bummer.


I put the 5881s back in and the 5V4GA and everything seems to be working great! Problem solved. 390v actual plate.

I had a pretty good suspicion of the recto's but nice you got it going. One suggestion is to make a simple light bulb current limiter and it could help save on fuses and more importantly IF the fuses didn't burn for some reason - your power tranny would be the thing next to blow. That's obviously not good.

Also, when I've made my limiter, use a GFCI with it too. Besides the bulb lighting up the GFCI will usually trip with a dead short, and it does so fast before the fuse will. Then you simply push it back in the re-set it once you go over your issues with the amp (which could be many and not obvious at first).

[daveyajd]

Thanks Jojokeo. I have a lightbulb limiter. I never even thought to hook that back up. The GFCI idea is great as well.

I did change out the cathode resistor to a 300 ohm. Current on the 5881s is now 55.4 mA.

Aaron