New Nightmare (Guitar Amp)

[Trout]

If you check the current (voltage across the cathode resistor) and it doesn't go above the recommended max for the transformer you are OK. The transformer that you are using claims to handle "80 mA per side" but it will probably sound better with 50 or so. I have used EL-34's in amps that have a bias adjustment , and found that some re-adjustment was needed.

If you try them, have a voltmeter connected across this resistor when you power up the amp. Divide the voltage reading by the value of the cathode resistor to get the total (both tubes) tube current. If you are below 120 to 140 mA total you are OK. Closer to 100 would improve tube life.

Example; You measured 25 volts across 220 ohms (with your 6V6's) That works out to 114 mA total. You can adjust the tube current by playing around with this resistor value (remember to divide by the NEW resistor value). The tube current affects the tube's tonal characteristics and its life expectancy. Unfortunately the sound that I like (and many other guitarists) seems to coincide with a short unhappy tube life. Just below the melting point. My next guitar amp (a really big SE amp) will have this as a front panel control. That way I can crank up the tube only when I need it.

I noticed in your original Fender schematic that the cap across this resistor is only rated for 25 volts, this is marginal for a guitar amp, due to the likelyhood of extreme use. When this cap fails it can take the output tubes, and sometimes the transformer with it.
Fender was trying to save some money. Put a 50 volt cap there when you have a chance. By the way the value of this cap affects the bass response and the tonal character of the amp. You can experiment with smaller values to kill some bass.

Great Tips,

I had already used 50V caps instead of the 25's shown on the schematic. I had read about that on a post here in this forum before. I try to read nearly everything posted here by everyone both for learning more & of course knowing I will most likely be confronted with several already posted problems. My luck seems to head that way sometimes

 

[anatech]

Hey Gene,
Don't consider yourself a bother at all.

1. It's interesting and informative to many.
2. It's good troubleshooting practice for some of us.
3. We get to see a project come together, or get stalled.
4. It may encourage others to try where they may not have before.
5. I think we all learn something from what goes on.

So there you go. I enjoy watching these come together. Later I plan to build my daughter a practice amp. Later she may want something with more power, and I'll build that too. I will refer to your projects just to see the journey.

-Chris

 

[Tubelab_com]

I think it is good to see someone chronicle the experiences (both good and bad) of amp building. I know that I went through many of the same experiences when I started building amps, I just don't remember most of them. Many of my first amps were guitar amps, often Fender or Marshall clones. I never had the patience that you have, and if things didn't work relatively quickly, they got torn appart. Most of my early amps were built from discarded HiFi's or TV sets. These were very common in the 60's and early 70's. Unfortunately they were discarded for a reason, and not all of the parts were good. I guess this still happens, as you recently found out.

Reading your stories is convincing me to build another one of my "turbo Champs" since I found the schematic. It has been a few years since I have heard one of them.

 

[poobah]

Hey Trout,

I second these guys. I work with stuff all day that really hurts my head sometimes. My current project has 22 microprocessors, 2 computers and about 2200 (really) other parts... all surface mount crap that I have to wear eye loupes and magnifiers to work on.

It's a total kick in the pants (self-serving ego-boost) to help someone along!

In case you haven't already, go to www.duncanamps.com. There are 2 nifty programs you can downlowd there for free. One will show you what your tone stacks will do before you build them... very cool. Also very cool, is the power supply modeling program... it is very well executed. It will take you some time to master; you'll learn alot along the way though.

Keep it going,

 

[ivegotmono]

I,m a newbie and I have been reading this thread since it was put up some days ago. I learned more from this thread than everything else I've read in the last 3-4 months.
Thanks
Steve

 

[PB2]

Gene,

You seemed concerned about the 400+ plate voltage on the 6V6s. RC-25 has a Vertical Deflection amplifier application where the pulsed peak plate voltage is 1200V for the 6V6. So it's not as if the tube will blow up with over voltage. The plate dissipation rating is important however and it is wise to lower the bias current.

Mind if I ask if those tubes are made in USA old stock, or something newer? We want to know more.

Pete B.

 

[Tubelab_com]

The maximum plate voltage rating is usually conservative, but there is a point that, if passed, the tube will go into an uncontrolled runaway condition, regardless of negative grid bias. For many new production tubes that point is at or near the max spec. I have seen some Sovtek 300B's that will "avalanche" below their published 450 volt maximum, while others will take 500 volts. I suspect that this has something to do with the quality of the vacuum since air molecules inside the tube will ionize causing extreme conduction.

The pulse rating is quite different from the continuous rating, and has more to do with the ability of the tube not to arc. Keep in mind that if you are running 400 volts DC on the plate of an output tube, the peak AC voltage can go to 800 volts in an undistorted audio application. In a guitar amplifier driven to hard clipping the peak voltage can go much higher, especially if the speaker load is mismatched. That 1200 volt peak spec is not too far off if you treat a guitar amp like I do.

That being said Fender (and others) routinely fed 6V6's and 6L6's more than their published ratings. This is one of the reasons that guitar players change tubes so often.

 

[Trout]

Gene,

You seemed concerned about the 400+ plate voltage on the 6V6s. RC-25 has a Vertical Deflection amplifier application where the pulsed peak plate voltage is 1200V for the 6V6. So it's not as if the tube will blow up with over voltage. The plate dissipation rating is important however and it is wise to lower the bias current.

Mind if I ask if those tubes are made in USA old stock, or something newer? We want to know more.

Pete B.

All My Amps with 6V6's use RCA Nos. All My 6L6's are sylvania,
& GE.

The only new stock tubes I own are EL34 J&J's and a few 12AX7EH's. Generally I run my EH's for recording and practice.

I have Bugle boys and Mullards, But I rarely put them in my amps for anything more than references.

The Amp seems biased well on the 6L6's so I will most likely just leave them that way.

Time for another project

Gene

 

[PB2]

I was thinking of the 10,000 tubes or was that a joke that I missed?