Pro Junior IV Bias Mod and New OPT

[tjl161]

After reading about the hot bias in Fender Pro Jrs, I measured the bias on my PJ IV and found that V3 was 9.6 watts of plate dissapation and V4 was 10.1 watts. The EL84 is rated up to 12 watts, but 70% of that is what I was aiming for to extend tube life. I added a 50K trimpot to make the fixed bias in the PJ adjustable after seeing that mod online.

I'll do this in two posts since it will be a lot of pics.

Ok, time to open it up.







I replaced the resistor at R29 with a 50K trimpot. You need to connect leg 2 (wiper) to leg 1 of the trimpot for it to act as a variable resistor. I set it a 17.7K ohms before installing it to get me close before setting the new bias.



The trimpot does not drop right in. You need to use jumper wires to get it to go into the original R29 holes in the board.



While I was in the chassis, I wanted to add a new output transformer with both a 4 and 8 ohm tap on the secondary side of the transformer (more on that it the next post).

I was able to get V3 to 8.3 watts and V4 to 8.8 watts by using the newly installed trimpot. V3 measured 168.1 ohms across its half of the OPT, -4.23 volts from the center tap, giving 25.5 mA of plate current resulting in 8.3 watts of plate dissapation. The numbers for V4 were 150.9, -4.02, 26.9 mA, and 8.8 watts.

End of part 1

 

[maxvintage]

Looking forward to your second post since R29 is not visible in the first post and I can't tell what's going on with the trimpot!

Thank you for posting, BTW

 

[tjl161]

Part 2

While I had the amp apart, I wanted to add a new output transformer with both 4 and 8 ohm taps on the secondary side. This will allow me to use the internal 8 ohm speaker and an external 8 ohm speaker cabinet.

Old and new OPT (Classic Tone 40-18045)



Original single speaker jack.



Added a second speaker jack and a SPDT switch to change between the 4 and 8 ohm leg of the OPT.



I wired the speaker jacks in parallel so when both 8 ohm speakers are plugged in it will give a 4 ohm load. The switch allows me to go to 8 ohm if I'm just using the internal speaker.





The final product. Sounds huge and awesome.

 

[tjl161]

Looking forward to your second post since R29 is not visible in the first post and I can't tell what's going on with the trimpot!

Thank you for posting, BTW

I already removed R29 in those pics. The blue trimpot is sitting over top of where R29 was originally.

 

[Wharfcreek]

I just acquired one of these amps, and I'm really curious about the way people have 'modded' the thing to add some 'adjustment' to the bias level. Unfortunately, everything I'm reading is all about adding a trim pot to the bias supply side, but there doesn't seem to be any consideration for the variation between tubes. I guess the assumption is that in today's world the ability to purchase a 'matched' set of output tubes is pretty easy. Yet, I've been buying tubes from vendors all over the planet for decades, and I've yet to find any vendor that sells a set that are within a ma of each other. For the most part, all the ones I've purchased have varied by 4 to 6ma pretty consistently. This isn't as critical here in the guitar amp world, but in the HiFi world that imbalance leads to some measurable distortion that's an unwanted aspect of performance. Anyway, as I see it, these amps need both a trim pot for adjusting the bias supply to a desired level, but also a trim pot to adjust 'bias balance' between the two tubes.....or so I'd like to see. Maybe not necessary, but certainly seems like it would be beneficial to the amp's overall performance.

Thoughts? Tom D.

 

[Lowerleftcoast]

Maybe not necessary, but certainly seems like it would be beneficial to the amp's overall performance.

It is handy especially if using tubes that are not a matched set. Generally, with guitar amps, some mismatch is desired so it is not critical to match tubes exactly.

 

[Wharfcreek]

Yea, I've kind of accepted that 'mis-match' as being almost a functional necessity. So, I guess just getting the overall bias at the proper level is really the objective. But, again, here's another rub: Seems like everyone is going about this by simply 'adjusting' the negative voltage at the bias supply point, but no one is actually checking the bias level at the tube. When I do repair/reconstruction on HiFi amps that are 'fixed' bias, I generally add 'test points' by breaking the cathode-to-ground connection and installing a precision 10 ohm resistor. This way I can measure voltage across those resistors and get actual current draw measurements for each output tube. In reality I suppose this then ought to be compared to measured Plate and screen voltage reading, and further math conducted to be able to calculate what the precise current draw level should be. But, in something like a Scott integrated amp, the H H Scott company published a current 'spec' for each output pair......and in something like the 222C/D amp (which used 7189s) the indicated that the tube pairs should run at 44ma. So, 22ma per tube. That, by some standards, is pretty low! But, they run that amp at 410V or so at the plates, and 395V or so on the screens. So that's substantially higher than the 315V or so at the plates on the Pro amp. I've noted that the Schematic for the 'F' version of this amp doesn't have an actual voltage listed for pin 9 of the output tubes. They show a 'Y' connection of the power supply that is listed as having 294 Volts......which is the screen supply. But, it goes through a pair of 100K resistors before reaching the tube.......

Anyway, it's a great amp, and I'm having fun playing around with it. After a few more 'mods' I'll feel pretty confident in it's reliability down the road!!

Thanks for the help!!

 

[Lowerleftcoast]

I've noted that the Schematic for the 'F' version of this amp doesn't have an actual voltage listed for pin 9 of the output tubes. They show a 'Y' connection of the power supply that is listed as having 294 Volts......which is the screen supply. But, it goes through a pair of 100K resistors before reaching the tube...

Those screen grid resistors are 100R not 100K but that is a common typo.
Other than that, your observations on the Pro Jr have made me wonder about those 100R screen grid resistors. Maybe they serve a dual purpose. The typical Fender screen grid resistor is 470R. Many have speculated they should be 1K or so. Since they are 100R they are hardly of use ime, but could easily be used like the 10R resistors you installed on the cathode to get bias numbers. The voltage drop across the 100R could be used to derive the screen current. iirc BillM mentioned the OT on the Blues Jr was almost always at the 100R measurement mark and the voltage drop across the coil was used in a similar way. In other words the math could be done in your head since the resistances were always 1R, 10R, or 100R.

Seems like everyone is going about this by simply 'adjusting' the negative voltage at the bias supply point, but no one is actually checking the bias level at the tube. When I do repair/reconstruction on HiFi amps that are 'fixed' bias, I generally add 'test points' by breaking the cathode-to-ground connection and installing a precision 10 ohm resistor.

Yeah, I am one of the *old school* guys. I measure the voltage drop across the OT to get the actual plate dissipation. Not many of the shock brothers do that. In all actuality the number does not need to be precise so there is no wrong way. I do think it should be disclosed some of the methods for finding bias numbers include the screen current though.

Comparing guitar amps to Hi-fidelity audio gear is not right headed. The guitar amps need to violate the *rules* to get the harmonics and distortions guitarists expect. Full range is out the window. Flat response is out the window. Manipulating the waveform differently on either side of the cycle is desired. The new guitar amp modelling devices work better with Hi-fidelity audio gear imo but that is about it.

 

[murrayatuptown]

I am working on a friend's PJ (III/Rev E.?). I WAS going to try making each EL84 bias adjustable but abandoned the idea. I also abandoned multiple 'improvements' to the whole bias supply that caused more headache than improvement.

I decided on a rear-panel pot in series with the 15k (USA, 60 Hz) R29. I put a resistor in parallel with the pot in case the wiper ever opened up (different scenario than the all-series textbook example like Valve Wizard's). Actually, in this circuit, R29 is shunting some of the 'raw' bias so it's a combination series/parallel voltage divider. If the wiper opens, the bias actually becomes much more negative instead of disappearing as it would in an all-series bias divider. In simulation it would go to somewhere between -80 and -90 VDC. Might not bother the EL84's but it sure would bother the 35 VDC bias filter caps. So my parallel resistor let me set the range of the bias pot and keep the 35V caps from blowing. (I had already put the circuit board back in. Otherwise I would have used some 10 uF 250 V caps I had).

I didn't want to get too hung up on figuring out the plate and screen currents from the cathode current (oh, I added 1 ohm cathode resistors & voltage jacks: Vk/1 ohm = Ik). So I measured the cathode currents with the adjustable grid bias set for the original bias level (-10.3 VDC), and estimated the total plate and screen dissipation from the JJ EL84 datasheet (at different voltage, etc, so it's a dart throw). I estimated the ratio of plate and screen current and concluded the screen dissipation had more safety margin than the plate dissipation. Since I was reducing the dissipation, knowing exactly what % plate dissipation wasn't all that important, compared to the inferno level it originally had.

I recorded the cathode current range over the pot range just made a mental note that my friend's 'matched' tubes are matched under one condition. Having adjustable bias for both tubes would have been much more difficult to fit and just make conspicuous the fact that they would NOT be matched under all conditions.

I have a notepad somewhere with the currents written down. Running at 70% (or arbitrarily 'much cooler') Pd, I care less about the match or obsessive measurement. Solved the heat problem. He also was torching the EL84's in his opinion. I estimated it was at about 108-112% (assuming a ratio for screen current) of datasheet limits. Just measuring the cathode current and using that as the plate current for calculation gives high values so its more conservative anyway.

I measured the plate voltage at the OT center tap and the EL84 anodes and only saw a difference of 4 V with no signal (set bias with no signal). Measuring at the anodes seemed to cause oscillation (barely audible to me in the garage, but the dog went nuts in the house). I'll mark the pot setting for 'original' setting as a red line.

Not done yet, but this part is a success.

 

[Wharfcreek]

Been a while since I've seen a post on this thread.....but glad to see it resurrected again. I generally get away from 'electronics' during the spring, summer, and late fall.....which to some folks is already well into winter! Murryatuptown, I'll mention to you that my mom's side of the family was from just up the road from you! My grandfather used to run the pumping station on Lake Shore Drive at what I seem to recall as being Michigan Rt 31? This was right on the shores of Lake Michigan just south of the city of Grand Haven. Dear departed grand-dad was an integral member of the team that built the Musical Fountain in GH. Good times for me as a kid...... and I certainly remember my Michigan Winters!!

Anyway, LowerLeft; I'm not sure about the use of the 100R resisters per Fender's design intentions, and I well agree that design aspects of 'guitar' vs 'HiFi' amps have s LOT of disparity!! BUT, that said, 'Output Tube Arcing' has got to be equally tabu in either. And, I believe those 100R resistors are often called 'Screen Stability Resistors' in the HiFi world, and placed there specifically to reduce and/or eliminate that symptom. I've also heard them referred to simply as 'grid stopper resistors'. There is an article by a good friend of mine on the Tronola web site, and it's an interesting read. I'll post the link to the opening introduction on the article, and within that is yet another link to the article itself. Truth be told, it's a bit over my head! But, I understand more of it each time I read it....which has been several times now. I get the theory.....and the application....just not fully digesting all the 'details'. Here Ya Go:

Techniques To Maximize Power Tube Life (tronola.com)

By the way, I will report some 'interesting' results on a project I completed early this spring involving a complete rebuild of an old Fender Blackface Bassman amp. The old eyelet board had been stripped of all parts on the 'Normal' channel side, yet the 'Bass' channel side still had parts on it. When I went looking for some kind of parts location chart, I found that the bass side matched one of Fender's model types, and the eyelets all lined up properly for a different model on the Normal side. This, I believe, rendered the amp as some kind of 'transitional' model combining circuits of two different amps on to one eyelet board! Interesting!! Anyway, after completion, I ended up stuffing a pair of 6V6 tubes into the output side and adjusted the bias down to about 28ma each on the 'matched pair'. This was not only using the 'wrong' tubes, as it was designed for 6L6 application, but also was biased 'cold' according to all charts on the subject when accounting for both plate and screen voltages. BUT.....this amp turned out to be perhaps the most 'harmonic' amp I've ever had cross my shop bench! Using just a single 12" 4 ohm speaker in a home made box, amp head on top, I was able to get more 'tones' out of this set up than anything else I have in my arsenal. Yes, I tried to buy the amp from the owner when done! No, he wouldn't sell it to me!! DAMN!! That said...the experience has made me 're-think' some of the amp projects I've done in recent years, and I'm now on the hunt to see if I can find an old 'vintage' Bassman output transformer so I can try to rebuild this thing one more time with similar results. AND, I've be playing around with 'cold bias' vs 'hot bias' as a function of generating some better harmonic response. So far, nothing concrete to say about that. Maybe in time. Would certainly appreciate any comments on the subject!

Tom D.

 

[dougstrum]

I have an early 1st edition Pro Jr.
The chassis pics look the same.

On my amp I change R29 from 15k to 20k to cool the tubes a bit. No perceptible change in tone.

How big of change did you notice from OT change?

 

[NTC]

Anyway, after completion, I ended up stuffing a pair of 6V6 tubes into the output side and adjusted the bias down to about 28ma each on the 'matched pair'. This was not only using the 'wrong' tubes, as it was designed for 6L6 application, but also was biased 'cold' according to all charts on the subject when accounting for both plate and screen voltages. BUT.....this amp turned out to be perhaps the most 'harmonic' amp I've ever had cross my shop bench! Using just a single 12" 4 ohm speaker in a home made box, amp head on top, I was able to get more 'tones' out of this set up than anything else I have in my arsenal.

Small note - with 6V6's and that OT, an 8 ohm speaker is a better match.

 

[Wharfcreek]

NTC, I would not disagree with that ....... 'theoretically speaking' that is. However, once again, I rather believe that some of the 'variables' come into play here, and in this particular case, it turned out that the moons, stars, and planets all just lined up to make this particular combination of amp & speaker sound amazing. And, I don't say that lightly. If I've heard it once, I've heard it a million times. It has almost zero meaning to me any more. Were it not for the fact that I've never achieved such harmonic response from an amp at nearly any level, I'd not make the statement. But this thing was like it was alive!! It was like watching a chemelion change colors, only it was sound that was morphing into poly-tonal bliss! It was magical. Unfortunately, it's like the proverbial bloom on a rose: The amp is now gone, and I don't have anything here that comes close!! I even tried an old amp I built nearly 30 years ago using another bassman chassis, but I think it has a super reverb OT in it......so, 2 ohms for a 6L6 pair.....or so I recall. Anyway, was fun while it lasted!!! At this point all I'm doing is attempting to relate this experience with that of working on some of the Pro Jr mods out there.......and suggesting that not all of them may result in a sonic improvement. Or....that sometimes doing someting other than theoretically 'proper' way can definitely have some great results!! Running a colder bias may be great for tube life, but might have a detrimental affect on sonic performance. Or, the other way around....which is why 'trying it' is always a good plan!

 

[LightningPhil]

It is a wee little OT in the Pro Junior. Would be interesting to hear how a sensible upgrade alters tone.

Used a toroidal one in mine. Apparently not the correct thing to do, but works well. It's huge compared with the origional and fives much better low end.

 

[CharvelBlue]

A toroidal in a guitar amp? Now thats interesting. Can you give any more info eg. brand, size etc, and pics would be great too! Like to see how it fit in there.

 

[Willie Johnson]

Part 2

While I had the amp apart, I wanted to add a new output transformer with both 4 and 8 ohm taps on the secondary side. This will allow me to use the internal 8 ohm speaker and an external 8 ohm speaker cabinet.

Old and new OPT (Classic Tone 40-18045)

View attachment 603619

Original single speaker jack.

View attachment 603624

Added a second speaker jack and a SPDT switch to change between the 4 and 8 ohm leg of the OPT.

View attachment 603625

I wired the speaker jacks in parallel so when both 8 ohm speakers are plugged in it will give a 4 ohm load. The switch allows me to go to 8 ohm if I'm just using the internal speaker.

View attachment 603626

View attachment 603627

The final product. Sounds huge and awesome.

View attachment 603628

Cool amp setup, but I really like that P90 Tele!