You're welcome, Justin. We appreciate your nice comments.

You're welcome, TA :)

Thanks, Tim :)

You're welcome. I'm glad you enjoy them !!

You're welcome, Stephen.....Amen :)

I'm a little wary of the "adjusting by ear", but it sounds like it really works for you, TJ.

+Tricko You're welcome :)

I've had good luck with regular Alpha pots from Antique Electronic Supply.

I fabricate each one from material I have on hand, Bob.

You have just the right attitude, Peter. Keep on learning....but don't take chances with jobs beyond your current level of understanding. I would be honored to sign your diploma. Keep up the good work. The world needs more trained techs :)

@@UncleDoug Last March 2021 I purchased a 3rd. new Marshall JTM 45/2245 tube amp and it also had problems within a few day's so I returned it. I'm thinking that my home may have grounding issues. I do run a Variac and use a voltmeter to keep the voltage at @117-120 vac. Funny how none of my other tube amps have never had even a single issue ever, but I'm 3 for 3 with Marshall JTM45's having problems immediately after purchase (new). I definitely need to have a better grasp about tube amplifiers. To my ear the JTM 45 sounds so good that I want one, still want one. I need to learn how to take care of it myself. If I can do that then and only then I shall forward my Degree for your signature! - Peter age 71, 72 in 17 day's. P.S I'm going to change the 6L6 power tubes to 5881 power tubes in my '59 Bassman LTD (Note: the LTD in the name refers to "Lacquered Tweed" and nothing more). Also perhaps I should change the 5AR4 rectifier tube in my '59 Bassman to a GZ34 like in the JTM 45. I have also heard I should set the bias to 37mA. Lastly I read to pull the V1 preamp tube. I'm hoping to get the tone closer to the JTM45. When I compared these to stock amps the JTM 45 sounded better to me. I also heard that I should set the "Bass" knob on my '59 Bassman LTD to eight (8), for the JTM 45 tone. Is the term "P1 synonymous with "V1" valve position? I'm asking because I read that I should pull V1 and then also read to change the P1 to a 12AX7. This confuses me as the '59 Bassman LTD already uses three (3) 12AX7's. Also they mentioned pulling V1 but they didn't say to replace it with another preamp tube. I think this last bit of info is off base.

@@PeterDad60 Mastery of tube amp circuitry requires a lot of study and practice, Peter. You might consider buying an inexpensive, non-working tube amp and using it as a learning project.....then sell it for a profit and buy another. That's how I got started.

@@UncleDoug That's a great idea. Better than mine as I was thinking once again that you should open an "Official Guitar Amplifier Technical School", or maybe I could move to Arizona to be near you and then I could become your assistant and hopefully learn from you. Of course that last part is extremely selfish, inconsiderate and downright inappropriate of me. So your suggestion is not only timely but also a much better idea than mine were. BTW I'm an "air cooled motorhead" (motorcycles) as cars are an expensive hobby. Also the parts are smaller on motorcycles and they have less of them (V-2 vs. V-8 etc.). I went for my first ride of the year a few day's back and life is good! - Peter age 71 turning 72 in 16 more day's. Yes, I'm counting the day's

@@PeterDad60 We share another common interest, Peter. I spent years collecting and restoring vintage British motorcycles (BSA, Triumph, Norton). Until my kickstarting knee failed, I'd ride a different one every day. It's a shame how age encroaches on the activities that make our lives worth living.

+David Chatterton Thanks, David. I believe it was at least 1W. You have to be careful, if the bias adjustment knob is exposed, that it can't be accidentally bumped or moved. You might be better off making it a very short slotted-head shaft that can only be adjusted with a screwdriver. Good luck with your modification.

+Uncle Doug thanks uncle Doug I'm going to put the pot on the inside of the chassis without a knob so it can't be knocked cheers brilliant vids learnt a hell of a lot I'm an electrician in the uk and you ohms law and other tid bits are explain perfectly

+Uncle Doug missed some letters in that last post Just to add, I here your doing a vid about capacitors and was wondering what the method is to get the correct rating of capacitor in any given circuit I understand the voltage rating but not the farad Once again awesome vids

You're welcome, David. There are many different applications for capacitors in an amp circuit, and the uf requirements vary widely due to their individual purposes. I'll try to explain the different criteria in the capacitor series.

+Uncle Doug I wait in anticipation for the series Doug. Say hi to rusty and jack for me cheers

If the Filmosound has an NFB loop, then it can easily be made variable or switchable.

There aren't any cathode resistors, Michael, the cathodes are grounded. If you make the 47K resistors variable, it will affect the balance of each pair of output tubes. To regulate the bias voltage to the grids, you could substitute a 17K resistor and a 10K pot in series for R52. Be careful when you first turn it on to adjust the bias.....start with the pot at 10K to ground and gradually reduce the resistance while monitoring the plate current. Be careful !!

Thank you. I had a look and made the change, together with adding 1 Ohm resistor on each 6L6GC to ground, but... weird noise, smell of smoke... I stopped, then looked everywhere, nothing strange. switch on again.. weird noise, smell of smoke... I removed everything and back to normal. No noise, no sound even... power lights where very light, pale, no voltage on the green wire (should be 6.3V). Then, smoke from the power transformer.... done ! I don-t know why and how.... so I will look for a replacement of the transformer, this is an old 606430. thank you.

Thanks for all your nice comments and for the tip on the replacement photoresistor unit, Tom. Without a proper schematic, I can't offer any help on the biasing. The one I looked up shows cathode biasing.

@@UncleDoug Here is a link to a schematic and I welcome your opinion/comments on the biasing. Best Rgards & Happy Easter... music-electronics-forum.com/attachment.php?attachmentid=40393&d=1472138853

+Brett Reed You're welcome, Brett. Yes, as I recall, it was R40.....47K.

Thanks mate

You're welcome, Brett.

A fixed-bias amp does require a bias check each time that new tubes are installed, CRG. A cathode-biased amp generally does not.

Is it fixed bias (grounded cathodes) or cathode bias, CRG ?

Greetings, JC. We're all doing quite well.....thanks for your concern :) Since plate current (PC) and plate voltage (PV) are inversely related, reducing the negative DC grid bias to increase the PC will reduce the PV, and vice versa. I think it's this basic relationship, not anything to do with transformer flux, that causes the phenomenon you described.

Uncle Doug : thank you for that commentary! After review of my data sheets, they completely support your observation relative to mine. I should have come to that same realization/conclusion on my own. Just learned something more about the effects of the grid and screens in a pentode. What got me looking at this is the amplifier, double ended with (4) EL34, had a switch removing ground from the cathodes of the inside tubes. In checking the power dissipation with 4 or 2 tubes based ground removal I saw the 2 tube output reduce only 30%-40% instead of 50%. Understand now that while the cathode is removed there is still current flow from the grid to the plate acting somewhat like a triode in the ungrounded cathode tubes. Glad to have received your reply! Give Rusty and Jack a scratch from me. All the best,

That sounds like a method to reduce output power, JC, to make the amp more versatile. Our best wishes to you too :)

Uncle Doug : absolutely. It just surprised me that the power reduction was not 50%. Caused me to do some head scratching. Your first reply made me take a closer look. Plan on measuring current flow through the grid and screens to better understand what I see in actual power in the high and low switch setting. Also will take a close listen to the tone difference in high versus low setting as the bias in the low power setting becomes much hotter. The amp is currently still on my work table. May give some insight to why I always preferred the Marshall 50W versus the 100W head. This amplifier (Hiwatt DR-103, 1972 vintage) has always been used by me for its extremely airy and glassy tone at high volume in the high switch setting. Maybe used the low power setting once or twice a long time ago. Just recall there was little difference in volume and the sound was just much better in the high power setting.

Surprisingly, lower wattage does not correspond to reduced volume to the degree you would expect. If a 10W amp can provide a certain level of volume, a 20W amp is nowhere near twice as loud....actually more like 5 or 10% louder. To achieve double volume, you would have to greatly increase the power far beyond simple expectations.

Since your amp is cathode biased, the wattage would be a lot more than the pot in my conversion would face, since the conversion is for a fixed-bias (grounded cathode) circuit.....which is quite different. Unless you can find some high-wattage pots, I would not recommend this conversion for your CB circuit. You can use a pot to determine the ideal cathode resistance and then install a fixed resistor of that value, but I wouldn't use a pot permanently.

thank you for confirming that !

You're welcome, Hugo :)

A good choice would be Metal Film or Metal Oxide rated at 1W......the current and resistance are quite low, so I-squared x R is way less than 1W.

Could you provide the time in the video where you saw this, Gary ?

Lets' see....it would be at .24-.33 secs. What I'm hearing is that you removed the 47K resistor but I still see a resistor covering that gap that you're speaking of. If you removed it shouldn't it be gone? Excuse my ignorance, but am I missing something? Thanks.

I see now; I was thinking the capacitor was the 47K resistor that is bridging the gap. I just noticed the "+" sign on that cap. So the 47K res was never in the video. Sorry, I'm getting old and sometimes it takes me a while to process things.

The exact position of the 47K resistor is shown in the video starting at around 0:24, ES. This circuit is a Gibson GA-35 RVT. The schematic is readily available on the Internet.

Ok Thanks!!!

Could you please help me with an issue? I have a beautiful 67 Dual Showman Amp, and I was cheking the B Voltages. The B+ that goes to the output Transformer is 450V (as says the squematic), but when I checked the next B (after the choke going to the Screen grids), I found 430 V insetad the 445 V that reads int the schematic.. This lower voltage is due a malfunction of the choke? Or the 20 mdf - 525v electrolytic Cap to ground? Or an incorrect Bias un the output tubes? Thank you very much Uncle Doug, you are so great!! I'm very pleased to meet you throught youtube!!!!!

Ok, I found the 47K resistor in the schematic.. since this is grounded cathode, the bias is throught the control grid with negative V... thanks!

The values you have stated sound fine to me, ES. The actual voltages you find in circuits rarely match the schematic values exactly. Chokes don't go partially bad.....they simply work fine or not at all.

Gameing roulete I believe that any fixed-bias amp (grounded cathodes), with an independent negative DC bias voltage supply, could be modified so that the bias voltage was adjustable.

No, since the 5E3 is cathode biased. Thanks :)

No, it's a regular pot. The value must be selected to suit the bias circuit WC. Good luck with the Mesa amp. Hopefully, there are techs in your area who can work on it if anything goes wrong.

I too prefer schematics, SS. Thanks for the suggestion.

No problem, Bryan. I appreciate your input and am glad the videos are helpful. My amplifier knowledge is generally restricted to vintage tube amps, and I am not well versed in integrated circuits or active tone circuits. Perhaps you could do some Internet searches for information. Good luck !!

Yes I was thinking of doing this mod my Peavy 5150 so I can bias it. Was the mod like Uncle Doug's or another way? The pot that Uncle Doug used seems to be a great solution for me because I would want to be able to reverse the mod if I wanted to sell it. Would biasing the Amp help the Clean Chn you think?

I looked at the schematic briefly and it appears that replacing R112 in the bias circuit (at bottom left) with a combination of fixed and variable resistance that can go above and below the value of this resistor (which is smeared and cannot be read) would probably work. Good luck and be careful :)

Uncle Doug Thx Uncle Doug! You remind me of my Uncle Albert , & what American's were known for. I miss that America

You're welcome, W4.....glad to have been of assistance :)

Fixed bias amps often do have bias adjustment pots, while cathode-biased amps do not. "Fixed" seems like a misnomer......but unlike CB circuits, the bias setting does not fluctuate.

@@UncleDoug I'm sorry sir, I didn't say anything about trimpots or any other type of adjustable resistor. All I said was this...fixed bias amps are the only type of amp that needs to be adjusted. The entire reason they call them "fixed bias" is because you have to fix the bias. Unlike cathode biased tube circuits which are regulated basically by themselves in a closed loop. Adjustable bias amp=fixed biased amp. Cathode biased amp=it adjusts itself, you should not have to do anything to a Cathode biased amp circuit unless maybe the capacitor or resistor gets cooked. Which really should not happen in the 1st place. Watch a Dr. Z video. He explains the best. Also if guys knew the truth about Cathode biased amps & how the main reasons we use "fixed bias" circuits now being fixed bias circuits create more power than Cathode biased amps. Makes me think about car guys & how they always want more power. Those guys would want a fixed bias amp if they knew that. Cathode biased amps are great as well but since they produce less power or "wattage" as it's called. The Cathode biased amps are quicker to break up & push a tube into distortion. That's where I get lost though because you would think that an amp that creates more power would be quicker to push the tubes into break up/distortion but that doesn't seem to be the case.

@@MAP448 Thanks for your input, Matthew.

Thanks for your input, Serapio, but the Gibson GA35RVT amplifier is indeed a non-adjustable "fixed bias" circuit, in which the cathodes of the output tubes are grounded and the biasing is accomplished by applying a fixed negative DC voltage to the grids. This is the definition of "fixed bias". I added a potentiometer which allows the negative DC voltage to be adjustable, so it is now an "adjustable (fixed) bias" amp.....just like the video title (and your link) says.