Pro Junior IV - control plate and back plate are very hot

savbalac

I have a Pro Junior IV. After a short period of time, the control plate and back plate get very hot.

I know those EL84 amps get quite hot (I had an AC15 for 4 years).

1) Will the board get burnt or, worse, the amp catch fire?

2) Should I sell it and get a well-designed PCB amp (are there any?) or hand-wired amp (Fender Custom, Lazy J, Victoria, Morgan, Carr)?

I've had the amp for a year and 9 months. I use it once or twice a week. I like the sound and the size & weight.

ecc83

"very hot" is subjective, if you can bear your hand on it for a second or two it is probably at 50C or so and I would not worry about that.

Valve amps DO get hot! I had one shipped across the country to me with the complaint that the top control plate was "too hot to touch" but it was perfectly within  spec for power consumption and valve DC conditions. No polite way to tell the punter he has girly skin!

One thing to check. Look at the EL84s in a dim room. If there is any sign of 'red plating' even just a dull red glow down the middle of the anodes the bias needs checking. I have assumed the amp is cathode biased but will see if I can find out before ICBM pounces!

I's wrong! Fixed bias but not AFAICS adjustable. So, if it is red plating take it to tech to get the bias upped a touch. Or you could try another brand of EL84 but that is just taking a chance.

Dave.

savbalac

Thanks, @ecc83 - I'll look at the EL84s later.

The control plate can be touched for a while (it just feels hotter than the AC15 did).

I have read reports of burnt boards in older Blues Juniors. Maybe version IV of the Pro Junior won't have that problem.

snowblind

As I understand it Fender tends to bias these things way on the hot side. Doesn't do much for tube life but probably a nice little earner on spare valves for someone.

Not sure if the Pro Junior is affected in the same way but there can also be problems with a lot of the newer Fender amps whereby they burn the boards due to some poor choices with component settings and installation.

I'd suggest having it looked at to see if it is running hot and if yes have it modded to bring it back closer to the specs for EL84s in sensible operating mode.

thermionic

Is this an amp that would benefit from the “transformer tap fix”?

HAL9000

thermionic said:

Is this an amp that would benefit from the “transformer tap fix”?

As I recall - although you can do this on most Fender amps you can’t unfortunately do this on a PJ. Fender amps destined for Europe (including UK) are set to run on 230V; because UK/GB electricity is typically on the hotter side of 240V Fender amps will run hotter over here than in mainland Europe.

Modulus_Amps

thermionic said:

Is this an amp that would benefit from the “transformer tap fix”?

Yes most likely.

Export transformer has all the taps, as seen on the schematic

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Pro-Junior-Schematic.pdf

Getting the amps bias checked would be a good idea too.

@savbalac, if the heat is really bothering you you could get a low noise fan and install it to push the air out from inside the amp. I would get the mods suggested above done first though.

CaseOfAce

I just assume all tube amps run hot?

At the end of a 2 hour or so gig you can fry an egg on the back of my Hot Rod Deluxe. 

ICBM

Modulus_Amps said:

Export transformer has all the taps, as seen on the schematic

https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Pro-Junior-Schematic.pdf

Unfortunately - unlike all the other models - 230V is the highest setting on these, unless it’s been changed in the MkIV.

Modulus_Amps said:

Getting the amps bias checked would be a good idea too.

Biasing cooler is a good idea. The easiest way to achieve it without too much work is to add a resistor in parallel with R30 - or a trimmer, if you don’t want to be bothered calculating it or doing it by trial and error. A better way is to increase the value of R29, but that’s more difficult without taking the board out - although if you’re careful it should be possible to snip one lead and add an extra resistor/trimmer in series. I can’t remember the right values right now, but probably about 100K for paralleling R30 or 10K in series with R29 will be in the right ballpark.

savbalac

Thanks everyone. I guess it's running as it's designed to. Unless a problem occurs, I'll leave it alone.

p90fool

Also, don't tilt it back. Mine went from hot to the touch to untouchable if I leaned it backwards, due to the cabinet forming a heat trapping internal apex. 

PAL

As already mentioned a transformer tap may need doing if possible on that model. Fender export amps to Europe set at
 230volts in the UK our voltage is 240volts so they run hot ! I have done the tap mod on my 68 Custom Princeton Reverb
 & my Blues Junior and they run a bit cooler and I think they do sound better with less background noise. Hope this helps.

ICBM

p90fool said:

Also, don't tilt it back. Mine went from hot to the touch to untouchable if I leaned it backwards, due to the cabinet forming a heat trapping internal apex. 

It has to be said that the tilt-back legs on later Fenders weren't one of Leo’s best ideas, although though do look cool and make it easier to hear the amp.

PAL said:

As already mentioned a transformer tap may need doing if possible on that model. Fender export amps to Europe set at
 230volts in the UK our voltage is 240volts so they run hot ! I have done the tap mod on my 68 Custom Princeton Reverb
 & my Blues Junior and they run a bit cooler and I think they do sound better with less background noise. Hope this helps.

The Pro Junior doesn’t have a 240V winding, it can only be set for 100, 120 and 230V (and oddly appears to have an unused tap for 220V as well) unless the MkIV is different.

I’ve wondered if it would be possible to fit a Blues Junior power transformer, but I don’t know if the chassis cutout is the same size - and it’s a lot of cost compared to modifying the bias circuit, which gives most of the benefits.

PAL

ICBM said:

p90fool said:

Also, don't tilt it back. Mine went from hot to the touch to untouchable if I leaned it backwards, due to the cabinet forming a heat trapping internal apex. 

It has to be said that the tilt-back legs on later Fenders weren't one of Leo’s best ideas, although though do look cool and make it easier to hear the amp.

PAL said:

As already mentioned a transformer tap may need doing if possible on that model. Fender export amps to Europe set at
 230volts in the UK our voltage is 240volts so they run hot ! I have done the tap mod on my 68 Custom Princeton Reverb
 & my Blues Junior and they run a bit cooler and I think they do sound better with less background noise. Hope this helps.

The Pro Junior doesn’t have a 240V winding, it can only be set for 100, 120 and 230V (and oddly appears to have an unused tap for 220V as well) unless the MkIV is different.

I’ve wondered if it would be possible to fit a Blues Junior power transformer, but I don’t know if the chassis cutout is the same size - and it’s a lot of cost compared to modifying the bias circuit, which gives most of the benefits.

  I would contact Chris at Rift amps I'm sure he could help you.