EL-84 6G9 Tremolux issue

[Tonewood]

A friend brought an early 60’s 6G9 head w/GZ-34 and EL-84 power tubes over and and wanted me to hear a noise that it makes. We plugged it into an 8 ohm cab and turned it on. Let it warm up then turned on the standby. Loud noise, red flash & blown fuse. The flash was near the rect & power tube end of the amp. Checking the OT while still in the amp, the primary windings seemed fine, primary to secondary is fine but the secondary winding seems to be about 2 ohms. Does that seem reasonable? If that is reasonable, any ideas on what is the likely problem. I see that no screen grid resistor and almost 400 volts isn’t a great idea for an EL-84. I don’t have tubes to kill trying them in the amp and I don’t want to damage a transformer that’s not bad yet. Oh, and my tube tester seems to be on the fritz & I can’t see any tube issues visually. Any good/fairly easy next moves? -Thanks

[TigerAmps]

With the power tubes removed - does it blow the fuse?
Also, with the power tubes out check to see if you have a negative bias voltage and check the bias caps with a meter.
I would replace the bias caps anyway since they are very old. Also consider re-capping the power supply.
But first figure out why the fuse is blowing. Can you swap the tubes - they still can be an issue.
Can you check the power supply caps with your meter? If so, I would do that.
With an amp that is 40-50 years old, I seriously would consider an overhaul.

[olddawg]

If this unit is in stock condition and isn't trashed... It's a very rare and valuable amp, especially if he has the matching cabinet. I own a 1963 Blonde Tremolux with 2 original cabs. But it has 6l6 outputs. I've seen the EL84 versions sell on EBay for rediculously prices. $5k and up. I saw a clean one go for $12k a while back. Evidently they only made a few. If you are unsure of your skill level, I would strongly recommend taking it to a reputable, qualified amp tech. You don't want to be "learning" on this amp. You will make mistakes and probably damage a valuable collector's piece. My opinion anyway.

[DRH1958]

If this unit is in stock condition and isn't trashed... It's a very rare and valuable amp, especially if he has the matching cabinet. I own a 1963 Blonde Tremolux with 2 original cabs. But it has 6l6 outputs. I've seen the EL84 versions sell on EBay for rediculously prices. $5k and up. I saw a clean one go for $12k a while back. Evidently they only made a few. If you are unsure of your skill level, I would strongly recommend taking it to a reputable, qualified amp tech. You don't want to be "learning" on this amp. You will make mistakes and probably damage a valuable collector's piece. My opinion anyway.

Yes, exactly right. This is the only amp Fender made with 6BQ5/EL84 power tubes and only for a short while. You also might want to consider using a 5U4 or 5R4 rectifier tube to reduce plate voltage. With today's higher wall voltages, you might be over 400 volts on the plates which is pretty high for a 6BQ5 or consider going to 7189 tubes. Just something to consider.

EDIT:If you do want to work on it, consider using either a variac or bulb limiter for protection. There are threads on this forum about the bulb limiter or Google it. There are a few ways to do it.

[olddawg]

Yes, exactly right. This is the only amp Fender made with 6BQ5/EL84 power tubes and only for a short while. You also might want to consider using a 5U4 or 5R4 rectifier tube to reduce plate voltage. With today's higher wall voltages, you might be over 400 volts on the plates which is pretty high for a 6BQ5 or consider going to 7189 tubes. Just something to consider.

My "guess" is you have a bad rectifier tube or shorting filter caps. If it hasn't been played much there may be nothing wrong with the pre and output tubes. In any case, as I said before. If it is in good cosmetic condition and not too butchered up, I would replace anything defective with period specific NOS parts. A little pricy, but a good investment for this amp. If the modern line voltage is too much. Use a line voltage reduction transformer to plug it into. They aren't expensive.

[Tonewood]

Everyone-Thanks for the help. A lot of good ideas. Turned out to be a bad tube but I wasn't in a hurry to sacrifice taking out one of mine. The thing sounds great but the voltages are crazy.

I used TigerAmps suggestions & the bias and caps- all looked good. Then I stuck in an old 5V4 which should cut the voltage a bit. Turned it on & no problem. Checked some voltages & then stuck in some known good power tubes. All is fine except the crazy high voltage. I did all the testing with the variac set at 100vac and the 5V4 instead of the 5AR4 that drops less voltage. That put B+ at 400 & 398 on the plates and screens! The fils were about 10% low.

Not sure if I should put in a 5AR4 and let him go plug it into 120ac. That'll be a dang high B+. The other rect specs, even the 5V4 really aren't great for these voltages.

Olddawg's idea of a step down transformer seems like a great solution. Also, I like DRH1958's 7189 idea. Looks like the 7189A would be right on the edge.

Now that I know the value of this amp, I'm not going to work on it but I will pass along the ideas.

Thanks again!

[DRH1958]

The 5V4 shouldn't drop much more than the 5AR4/GZ34. Here's a chart with all the data you need. A 5U4 or 5R4 will drop more. And yes, dropping line voltage can help, just watch out for heater voltages(5 and 6.3) to be in spec.

Rect. tube info.txt

[Tonewood]

The 5V4 shouldn't drop much more than the 5AR4/GZ34. Here's a chart with all the data you need. A 5U4 or 5R4 will drop more. And yes, dropping line voltage can help, just watch out for heater voltages(5 and 6.3) to be in spec.

[ATTACH]40561[/ATTACH]

5U4 draws 50% more current than the 5AR4. I have a bunch of 5U4s but I thought that may tax the PT. 5R4 data sheets that I’ve seen show 4uF for input cap. Is that the Max? If 20uF isn’t a problem for the first cap after the 5R4, then that's a good choice.
Anyone know the max input cap for 5AR4? Is it bad to go up 50% in fil current? Thanks.

[DRH1958]

5U4 draws 50% more current than the 5AR4. I have a bunch of 5U4s but I thought that may tax the PT. 5R4 data sheets that I’ve seen show 4uF for input cap. Is that the Max? If 20uF isn’t a problem for the first cap after the 5R4, then that's a good choice.
Anyone know the max input cap for 5AR4? Is it bad to go up 50% in fil current? Thanks.

Good catch about the 5U4! I usually remember that. That's why I posted that chart so you could decide what tube to use and help on the research. The max cap size spec'ed on the data sheet is only if the tube is delivering its full rated load current which a 5R4 should not be doing with this amp. The 5R4GYB is rated for 20uF max cap size: http://www.tubebooks.org/tubedata/HB...bes/5R4GYB.PDF It's rated for 250mA and the heater is only 2A, same as the 5AR4. I have two 18 watt Hammond amp conversions I use that tube in to reduce B+ and it has a 50uF reservoir cap. I have had no problems with it. So this tube should fit your needs.

This data sheet says 60uF max cap size for a GZ34, same as a 5AR4: http://www.mif.pg.gda.pl/homepages/f...010/g/GZ34.pdf

I don't think it's worth the risk about the extra heater current for the 5U4 but Fenders were known for robust trannies. If you want to try, you can measure the 5 volts on pins 2,8. If it's low, that indicates that winding is being taxed. If it's at 4.9 or 5 volts, it's usually good.

It's too bad this is happening with older amps that ran the B+ over the power tubes spec voltage. It's a pick your poison type thing. If you lower the wall voltage, then you run into the problem of low heater voltages as you did with the variac set lower than 120V. It's best to try to lower the high voltage B+ only. Of course the 7189s will solve the problem too.