ampeg reverbrocket question

[mikhett]

Hi ,My name is Mike and this is my first post. Im an ET with the us govt and im good with electronics but new to tubes. I have a 66 ampeg reverbrocket with 6v6 tubes that originally hag 7591s.I rewired the tube sockets to make this work but didnt change the OT. The original trans had 6600 ohm output impedance so i figure the 6v6 s would work.This output circuit is cathode biased so i cant adjust without changing the 140ohm 10 watt cathode resistor ,right? The amp works but it seems a little weak. Do you think i need a fender 6v6 ot to make it louder. Thanks Mike

[mikhett]

btt

btt

[tim]

Hi Mike And Welcome!

I would suggest first checking the bias on your power tubes. This can be done easily with a voltmeter and a calculator...here are the steps:

1. Measure the voltage drop on the cathode resistor to ground (Basically measure voltage across the 140 Ohm resistor). Write the value down.

2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

3. Measure the voltage on the plates of the power tubes to ground. Write this down.

4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.

6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in.

After you get this checked out and remedied, then you can start looking elsewhere if it still sounds weak.

BTW, the brand of 6V6 will have very little if any bearing on volume of amp, unless of course the tube/s is bad to start with.

[mikhett]

Thanks Tim.

Thanks Tim. Its all coming back to me now. In the late 80s i did a lot of amp workand in fact my friend also mike a pro guitarist knew Ken Fisher of TRAINWRECK fame.Mike went to go see a group called BB & the Stingers and bb Bernie Braustein was playing a TW. He came back to me RAving like a lunatic about this TW amp, But he was just 20yrs old and couldnt come up with the 1500 for the amp. Thanks Tim

[tim]

I always thanked Uncle Sam for teaching me tube theory in Navy Avionics School...I was the last class that went through tube theory in 1975. After my class, they only taught it as "here is a tube and this is what it looks like...now moving onto solid-state". I had S/S as well but I have always struggled with it for some reason.

I've worked on a Train Wreck amp or two in my day...also many other very cool amps as well, most of them I could never afford. I just don't get attached to them as I work on them...fix 'em and get them out the door. I used to do the repair thing full time but now only do it part time several nights a week in my shop. I can now afford to be somewhat picky on what amps I work on these days...

[mikhett]

tim i used 6v6s back in the 90s and

the values i used were 10 watt 250 ohn k resister.I measure 22v at cathode,80 ma k current 343 p volts .i get 12.84 watts dissipated. Can i leave the k resister alone. I dont know if i will find a value so little higher. Thanks Mike

[tim]

That's a bit over the high side of normal but should be OK...you could always parallel two resistors to get closer but a bit more than 12 watts is probably OK. It'll break up a bit quicker (maybe a good thing) and the tubes might not last quite as long with it being biased on the warm side, but 6v6's are a darn sight cheaper than 7591s these days.

[Bruce / Mission Amps]

The OT isn't the issue.
7591s are more sensitive then 6V6s. They won't require as much drive to go to full output, so I'd suggest coming up with a little more drive out of the preamp to kick it in the pants and changing the cathode biasing resistor to 250ohms to 300 ohms. Replace the cathode bypass cap too. 22uF@50v to 100v up to 100uF @ 50v to 100v is all you'll need
If you aren't able to do that easily, just find a pair of 7591As and replace them.

[tim]

Bruce,

You are right...I forgot that the 7591's need less drive. I've only worked on a few amps with them used as power tubes. Most likely why he's not getting the same power out of the 6V6's...

Mike, I believe that Electro Harmonix has 7591-A's now. Not sure how good they are and how they compare to 7591's of yore but I think Antique Electronic Supply carries them. (www.tubesandmore.com)

[Gtr_tech]

JJ offers a 7591 now also. No exp with those as of yet.

[mike5623]

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