I want to convert 6L6's to 7591...

[Bmxbarspin724]

First of all, I love the sound of this tube. I love how it stays clean all the way up and then only get's a little bit of grit to it. It's just an edge.

I have a couple of 7591 tube related questions.

1.) I have 2 amps that both run with a pair of 7591's. the problem is, that one is a 25 watt amp, and one is about a 18, or so. I just wish I could have the tone and edge of a 7591 but with more power. About 40-50 watts.

I know that with some pin layout change, that you can sub a 6l6 for a 7591. I'm wondering, can i do it the other way around, and what would it take, mod-wise? I really am set on 7591, so don't just say, stick with the 6l6...

the reasoning behind it is, fender hot rods and devilles are really plentiful around here and a good bang for the buck, and I know they both use 6l6's.

Or maybe I could take an amp that had 4 6l6's and replace them all with 7591's...

That is my first question.


2.)I have an ampeg rocket 2 that uses a 7591 pair, 1 12ax7 and 1 7199. I also have a 1965 epiphone futura that uses a 7591 pair, 2 12au7's, 4 6eu7's and an OA2 tube regulator.

The epiphone is rated at 25 watts, and the ampeg is rated at like 18-20 watts. That seems reasonable considering that they both use the same power tubes. when I crank the epiphone all the way up, It stays clean! I am talking about all the way up, volume wise! I mean, It doesn't stay totally clean, but it doesn't distort, it just gets edge. and it's smooth edge. and it's much much louder then the ampeg. taking in the difference in speaker size too...

but when I crank the ampeg, even just to 2-3 o clock, it get's really fuzzy, and distorted. and it's not as warm, either... It seems very very cold in comparison to the epiphone futura. So I'm wondering, besides the epiphone having a whole slew of pre-amp tubes for the tremolo and the reverb that comes with it, what could be the main difference between these 2 amps?

could the ampeg be biased too cold?

3.) I had purchased a set of 7591 tubes in case either of them went out on my 2 amps, and sure enough the epiphone futura was making this wierd crackling noise whenever I would strike a chord hard, and so I switched in the new set of tubes I had bought. They were sovtek 7591's that I bought on a bad impulse while in my local music store. I have since come to find out that they are actually really close or identical to 6l6's, and that they can be used, but with a bias modification...

I had heard that any new power tubes will be matched differently then the NOS ones that had been in the amp before, and I just figured that i didn't care if the tube life was shortened by a little bit, if it sounded ok then. Now I am wondering if I am messing something up by using these supposed "6l6's" They aren't running that hot, but hot enough. They look totally fine, and if I hadn't read it online, I wouldn't have noticed a thing...

considering that I love the tone of this epiphone when it has "basically 6l6's" in it, I am wondering If i'm right in my quest for 7591-ness...

well, anyways...


I know that's alot, but thanks for reading, (if you did...) and thanks in advance for the answers that I hope I will get.

-Noah.

[Old Tele man]

...do the "Twin" trick and 'add' two more output tubes.

...just add one on each "side," in parallel with the existing tube, and then "half" the speaker load (another speaker?).

...of course, you gotta wonder if the OT can handle the added current and output power?

[Bmxbarspin724]

That seems overly simplified...


Could someone please explain that a little more in depth?

[stokes]

You are not going to get 40-50 watts out of a 25 watt amp without doing major reconstruction to the circuit.What Old Tele Man is suggesting is adding 2 extra power tubes to the amp,thereby doubling the potential output.The OT probably couldnt handle the extra power,so it wont get to your speaker anyway.As far as the wattage differences in the 2 amps,it is possible that they put out the same power,a lot of manufacturers take a little freedom in describing the amps wattage.What you are most likely experiencing is age.Your tubes are very old as are your filter caps,weak caps will cause the amp to break up quicker and sound weak.If you like the sound of the Sovtek 7591's,then just use them and dont worry about the fact that they are just re-pinned 6L6's.If you really want the sound of the 7591's,and by your own words,you dont hear a difference,find some NOS's,but get ready to shell out some big $$$.It is possible that the Ampeg is biased too cold,but I think it is more likely time for new filter caps and tubes.

[Bmxbarspin724]

as far as the sovtek 7591's go, I like them fine, but is there any damage to my amp when I'm using them, considering that you should do a "simple bias adjustment."


Will I be fine if I don't adjust the bias?





and I wasn't ever really talking about getting 40-50 watts from my 25 watt epiphone, I was wondering if you could take an amp that used 6l6 and convert it to 7591...

and no body really answered that question...

[Jazz P Bass]

6l6 -7591

A tube is a tube.
If you follow the parameters on the datasheet, then yes a 7591 will work fine.
The transcunductance looks the same, so you are good with your present output transformer.

[stokes]

You cant use a 7591 in an amp designed for a 6L6GC.The 6L6GC is good for 30 watts,the 7591 is good for only 19 watts max.A 6L6GC can be used in a 7591 circuit,but not the other way around.The transconductance isnt the issue using the OT,in this case,its the current draw.As far as doing damage to your amp using the current production 7591's,it should be fine as long as your plates arent glowing red.The bias should be checked any time you change power tubes,even when using tubes of the same type,but as long as it sounds okay and the tubes arent redplating,it is safe,perhaps not optimal,but safe.
"1.) I have 2 amps that both run with a pair of 7591's. the problem is, that one is a 25 watt amp, and one is about a 18, or so. I just wish I could have the tone and edge of a 7591 but with more power. About 40-50 watts."This is waht led us to believe you were trying to get more power out of the smaller amp.That is where Old Tele Man was going with the 4 output tubes.

[imaradiostar]

I've been a fan of the newer EH 7591 for a while now- not sure if the old ones are better for guitar or not but I really like the new ones. They're a higher slope tube somewhere between a 6l6 and an EL84 in drive requirements- it doesn't take much to make them sing. My main gig amp uses them and I regularly lend it out as a recording amp with positive results.

My recommendation is to double up the output tubes as previously suggested. Though they're an 18 watt dissipation tube they'll do more than that in push-pull- 40 a pair with no trouble. They'll perform better with a slightly higher reflected load impedance- I'm running mine at 6k6 and it seems ideal. It wouldn't be tough to use an edcor or hammond output tf with them and a twin reverb PT- it should easily be as loud as a twin but retain the 7591 sound.

I haven't looked at the schematics for the amps in question but it may be possible to make some minor changes and increase the headroom of your existing amps. Are they cathode biased? Convert to fixed. Is the sceen voltage substantially lower than plate voltage? You may be able to increase the screen voltage or trade out rectifiers to increase plate and screen voltage for a little more headroom. This may require work to properly bias the tubes and so on.

I just took a quick glance at the futura- you could use solid state rectifiers, increase the filter cap size and move the output center tap to the upstream side of the filter choke. This would increase your plate voltage 15-30 volts and reduce the amount of sag under load. You should probably slightly decrease the value of the 330k resistor in the bias supply- I would simply parallel large value resistors with it till the power tubes were idling at the appropriate current. I'd probably add 1 ohm resistors between the 7591 cathodes and ground so I could measure the current at each tube. That should be enough to get a pretty solid 45 watts out of that little amp.

That leads me to wonder- it should already be doing darn close to 40 watts since it's not far off from the values on the datasheet- if it isn't you might check the health of the filter caps and output tubes.

jamie

[Bmxbarspin724]

The epiphone is a 1965 epiphone futura, and it does use a solid state rectifier.

There are like three different versions of this amp, but I have found out that the gibson GA-30 crestline uses the same tubes and puts out about the same power. It was said somewhere that it's basically the same amp...



So basically, all I have to do is add 2 more 7591 power tubes, (which I would undoubtedly have a tech do) and then add a new power and output transformer and half the speaker load?

The speaker load is at 8 ohms now, and so I should drop it to 4 ohms?

Let's say I was to get a output transformer that had outputs for 4, 8 and 16 ohms, would I have to set it to 16 ohms and then use a 8 ohm configuration? or if I set it for 8 ohms, would I have to run a 4 ohm speaker configuration?


and thanks for all the help. I'm beginning to understand alot more.

[stokes]

If you plan to add two more power tubes (do you have the room?) and add a new power and output tranny,the speaker load will be dependant on the turns ratio of the new OT,just select it to match whatever speaker load you want to use.

[Bmxbarspin724]

I guess I'm wondering a few things...

and on each side of the power tubes there is one 12au7. I was wondering, could I move those 12au7's about 2 inches up and away from the power tubes to make room for the 2 new power tubes that might go there? assuming there is room inside of the chassis for the rewiring, does it matter that the tubes are not all in a line?


and how much should I expect wattage wise out of 4 x 7591's?

19 X 4 = 72 watts? that would be cool...

Is 70 watts realistic?

[imaradiostar]

You should try some more efficient speakers to make the current amp a little louder- call Weber and they can make some recommendations.

I think you should seriously consider building a higher wattage amp in a new chassis before "upgrading" a functional amp. It's easier than you'd think, especially with the wealth of information you can find by searching the forum archives here. My only warning would be that an amp with double the output power will only be a few dB louder- wattage ratings can be deceptive! It should be pretty easy to get a similar apparent increase in volume with a speaker change.

jamie

[Bmxbarspin724]

You should try some more efficient speakers to make the current amp a little louder- call Weber and they can make some recommendations.

I think you should seriously consider building a higher wattage amp in a new chassis before "upgrading" a functional amp. It's easier than you'd think, especially with the wealth of information you can find by searching the forum archives here. My only warning would be that an amp with double the output power will only be a few dB louder- wattage ratings can be deceptive! It should be pretty easy to get a similar apparent increase in volume with a speaker change.

jamie

I plan to make this amp into head form and run it into a 4x10 cab and a 4x12 cab. Which would sound awesome.

I really love the sound of this amp, everything about it. I just love it. but I would like to have more power. I mean, to me, 25 watts (or even 35. whichever one you believe) is alot different than 70 watts. especially with 7591 tubes since they stay clean so far up.

and I don't think it would be too hard to rearrange a few things and have a tech put in 2 more power tubes.

I'm planning on using this amp as my main gigging amp, that's why I want more power. granted it's only a few decibels, it definitely sounds louder, and that's all that matters, right? sound...


I've really appreciated all the help and information I've got here.

[soundmasterg]

The 7591 can make a lot more power than it does in those vintage circuits. Probably for economic and product placement reasons, makers who used the 7591 would run them at lower voltages than what they can really do, so that they would not have as much power as the 6L6GC amps, and therefore would cost a lot less with less performance rather than costing a lot less with almost identical performance.

The 7591A can take 550v plate and 440v screen and is a 19 watt max dissipation tube. So with 450v plate and 400v screen, fixed bias, 6600 ohm load, you can get 45 watts out of a pair of 7591's. They'll sub into your 6L6GC sockets if you rewire the sockets appropriately. You would just have to make sure to not bias them as hot as the 6L6GC's in whatever amp you sub them into.

Another consideration is that the 7591 has more gain than the 6L6GC, so if you sub them into a 6L6GC circuit, the preamp may need less gain.....but it all depends on what you want. They're one of my favorite power tubes.....

Greg

[stokes]

"I don't think it would be too hard to rearrange a few things and have a tech put in 2 more power tubes." Its not as easy as you seem to think,and will likely cost a lot.