My First Amp Conversion 22 Years ago, General Electric PA-20 Mono Hi-Fi

[sluckey]

What about R25(680/853) and 28(1800/2100). I think I could access R28 fairly well with extreme caution but R25 is a pure bugger

Those are cathode bias resistors. Slightly larger value means the tubes are biased slightly cooler. No big deal in a preamp stage. I'd let it be.

[Platefire]

Oh Good! That should wrap up the pre amp section. Got some resistors in the power amp section I need to take a secondlook at. I need to get it back together while I can still remember where all the panels and screws go at. I want to take
some finish pictures gut shots and assembled shots.

Oh yeah, this amp has a hum balance pot hooked to the heater wiring. I just happened to check the heater voltage theother day and one side was 3.8 VAC and the other was 2.8 VAC. So I adjusted it where it was balanced at 3.3/3.3
Thanks

[sluckey]

Oh yeah, this amp has a hum balance pot hooked to the heater wiring. I just happened to check the heater voltage theother day and one side was 3.8 VAC and the other was 2.8 VAC. So I adjusted it where it was balanced at 3.3/3.3
Thanks

A better way to adjust the hum balance is just turn the pot for minimum audible hum. If your ears can't decide then connect your AC voltmeter across the speaker and adjust for minimum voltage.

[Platefire]

uh oh, got a problem! On the V3B Phase inverter grid pin 7, I got about 40VDC. Schematic says -2VDC there.

 I know I'm suppose to have 65V on the cathode which I reading 78VDC up through the 3K cathode resistor where it is then 76DCV up to the 470K. Then on the other side of the 470K it turns to the 40VDC that is on the grid.

So I'm thinking C12 .047uf is leaking. Don't know where else it could be coming from?

[sluckey]

The -2V on the schematic is wrong! Nothing is wrong with the amp.

[Platefire]

Well Thanks, that's good to know. I'll ride on that 

[PRR]

Weller 8200N - Is that one of those .....

[Platefire]

Yeah, PRR that's just like mine but it appears yours is in better shape than mine. Are you having any trouble finding solder tipsfor yours. I just installed my last spare tip a while ago and the last one I bought was from Radio Shack and they are closed now.So I hope there are other places to pick those up at

[sluckey]

The -2V on the schematic is wrong! Nothing is wrong with the amp.

I should clarify... There's nothing wrong with the voltage reading on the PI. Of course there may be other problems in the amp.

Yeah, PRR that's just like mine but it appears yours is in better shape than mine. Are you having any trouble finding solder tipsfor yours. I just installed my last spare tip a while ago and the last one I bought was from Radio Shack and they are closed now.So I hope there are other places to pick those up at

Amazon...

[bmccowan]

Weller 8200N - Is that one of those .....

Yup - I've got one tucked away somewhere. Was my Dad's. I've used it a few times remelting chassis solder puddles. Hey if it works for you, Platefire, go for it. I think I'd be burning wire insulation and film caps even worse than I do now.

[sluckey]

That 8200 is also useful for cutting synthetic braided rope using the proper tip.

https://www.amazon.com/Weller-RCT-Rope-Cutting-Black/dp/B0002BSQR6/

These tips are very easy to make with a bit of 12AWG solid wire, a hammer, and a vise. Most marine supply stores have an 8200 in the rope section.

[Platefire]

I understand sluckey on the PI voltage. One crises averted, looking for the next one
Thanks also for the source on the 8200 solder tips.

bmccowdan--I have burn up a few things with that 8200. You have to be easy on trigger and keep that tip clean

[PRR]

...yours is in better shape than mine.

Sorry-- that's grabbed from eBay, just to remind me what a 8200 is. (I have two Guns, neither an 8200; I know Weller made many variations.)

[bmccowan]

That 8200 is also useful for cutting synthetic braided rope using the proper tip.

https://www.amazon.com/Weller-RCT-Rope-Cutting-Black/dp/B0002BSQR6/

These tips are very easy to make with a bit of 12AWG solid wire, a hammer, and a vise. Most marine supply stores have an 8200 in the rope section.

I rebuilt a 20' wooden sloop many years ago (doing this will make you think the amp hobby is super cheap) and the local marine store had just that - and they let the customer use it themselves! I wonder if that still happens?

[PRR]

...marine store had just that - and they let the customer use it themselves! I wonder if that still happens?

The Hamilton Marine in Southwest Harbor might. It's way off the highway, not many jerk landlubbers find it.

The Hamilton in Searsport is right ON Rt1 next to the flea markets, so customers can't be running loose in there.

Gotta respect a company which blends their own Lobster Buoy Paint and then posts a Stress Test with wire brush, scraper, files, and parking-lot.
YouTube "Lobster Buoy Paint Stress Test"
For away folks: the modern Lobster Buoy is Styrofoam, so it is amazing any non-toxic paint even sticks, much less takes abuse.
Catch the Mount Desert Water plug at 2:36.

[bmccowan]

Love that video, especially the parking lot test. I have a pair of oars that need painting - that paint might be just the ticket.
“Believe me, my young friend, there is nothing - absolutely nothing - half so much worth doing as simply messing about in boats.”
― Kenneth Grahame, The Wind in the Willows

[Platefire]

Trying to calculate bias for this cathode bias amp. So here goes:
I=v/r,  I=Voltage across cathode R/Measured cathode R Ohms,   I=26.16VDC/241Ohms,    I=.10818,   .10818/2 tubes=.05409 

54.09 DC Milliamps Bias
am I even in the ballpark?

It's got Sovtek 6L6WXt's

[sluckey]

You're in the ball park. More specifically, you're on second base. Gotta do a little more math to get to home plate.

[PRR]

Trying to calculate bias for this cathode bias amp.

Why?? Do you think GE got it wrong? Make Rk equal 250 ohms and quit fretting. (Or start fretting, your choice.)

[Platefire]

Well PRR, no doubt the cathode resistor is correct, I'm just curious. What little I been studying on cathode biasing, it appears that the mA are normally set higher/hotter than fixed. I really slow on this math, so it don't hurt to give it a run every once and a while

I think what sluckey was wanting me to do was calculate the wattage and also the dissipation to reach home plate

My plate voltages are 440/436. so averaging those out is 438VDC. So to calculate power P=IV, P=438 X .05409=23.69 Watts

I think my little ol brain is fried, hafto come back to this latter

[sluckey]

I think what sluckey was wanting me to do was calculate the wattage and also the dissipation to reach home plate

My plate voltages are 440/436. so averaging those out is 438VDC. So to calculate power P=IV, P=438 X .05409=23.69 Watts

Well, now you're on third. That 23.69 watts you calculated was for the tube and the cathode resistor. But you need only the tube dissipation in order to reach home plate. So, subtract the cathode voltage from the measured plate voltage to get the voltage across the tube. Then multiply the voltage across the tube times the calculated cathode current.

[Platefire]

OK, I think go get cathode current I need to divide the measured voltage at cathode by the measured value of the cathode resistor

So 26.16/241.8=.108

so to get % of dissipation multiply voltage across tube 411.84 X calculated cathode current .108 =44.48 % dissipation

Well, am I home yet?

[sluckey]

44.48 is static dissipation in watts. I don't know what "% of dissipation" is. And 44.48W is for two tubes.

Well, am I home yet?

It's a close call. What about it judges? Shall we give it to him? 

[72Blazer]

  44.478 

Looks good to me

Thanks for the lesson...again

[Platefire]

Sooooooo-44.48 Static dissipation in watts. Trying to figure what kind of information I've got with that? Does this relate to the totalMaximum Plate Dissipation in the Tube specs?

[72Blazer]

This is purely a guess, and Steve (or other really smart guys here on this forum) will correct, but I think it relates to the efficiency somehoe of the tube/how it is performing.  Again, a guess.  Heck, I'm on my 2nd bourbon now-and listening to a Pink Floyd (David Gilmore) concert-and he just pulled out the BLACK STRAT!!... 
; )

Vr
J

[Platefire]

Well I know on fix bias amps we set the bias at 60 to 70 % dissipation to get the most efficient tone/sound but at the same time not burn up the tube because if it was set at 100% dissipation, it wouldn't last too long. So I'm thinking this is related to the same thing---but don't recall put it in these terms "static dissipation in watts".

I'm thinking it's the way of measuring the amount output the amp is producing at idle with no guitar signal??

[sluckey]

Don't let the word "static" confuse you. Static simply means at rest. No signal applied. Maybe I should have used the word "idle". Means the same thing.

Yes, it relates to the max plate dissipation seen in the tube charts. For fixed bias many of us like to set the static/idle dissipation at about 70% of max dissipation. For cathode biased amps the static/idle dissipation typically runs about 90% to 110% of max dissipation.

[Platefire]

OK thanks for the clarification. So we have some calculations and figures. So let me get at the question I wanted to know all along. On this cathode biased amp, what is the % of dissipation this amp is set at? or what calculation we have already done will tellme that?

[sluckey]

The % of dissipation usually is the ratio of your measured idle dissipation to the tube chart max plate dissipation.

You measured/calculated 44.48W. But that's for two tubes. One tube would be half that (if the tubes are perfectly matched). So let's use 22.24W for one tube. And JJ 6L6 says PaMax is 30W. So...

     22.24\30 = 74.1%

However, most tube charts say the PaMax is only 19W. So, in that case...

     22.24\19 = 117%

[Platefire]

Thank you so much!! This brings me to the home base I was wanting to reach for myself on this amp. Learning more than I ever have on cathode bias. BTW-does PaMax mean "Peak Maximum"? Using the 19watt reading seems pretty hot but on the other hand, I have had those Sovtek 6L6WXT 22 years in this amp and they are still kicking pretty strong. I paid $20.00 for the set back in 2001. So I'm glad we got this all documented, on hard copy and also on this forum, so I can go back and grab it when my forgetter gets to working real well.

Guess I will push on to doing final checks on the power amp.

[sluckey]

PaMax means Power Anode Max. Anode is same as plate.

[Platefire]

I've got 5 resistors in the power amp that are reading out of spec or more than 10% off and no problem because I've got replacements for 4 except for the 3K cathode resistor R37 PI V3B that is reading 3448 in place but it appears to me it does have a 470K in parallel to it plus a 110K in series to ground with it. So that may be whats throwing it off. Physically it don't look burnt and colors is good.

I will lift a leg and measure it that way before I make a special order for that one

[sluckey]

There's nothing in parallel with R37. You can accurately check it in place. I'd let it be unless I was super bored.

[Platefire]

I'm not super board I got burgers to grill and got to run to Wally world to get some ice cream salt and ice. Hope they are open?

OK to be officially in parallel, the resistor would have to be connected to the resistor in question at both ends. Right? One end
don't count

[sluckey]

I'm not super board I got burgers to grill and got to run to Wally world to get some ice cream salt and ice. Hope they are open?

Don't forget the watermelon!

OK to be officially in parallel, the resistor would have to be connected to the resistor in question at both ends. Right? One end don't count

That's right. And the parallel connection may not be obvious. Usually (but not always) cathode or plate resistors can be checked in circuit because one end terminates at the tube pin with no other resistors connected, as is the case with R37. Always helpful to have a schematic handy when checking resistors in circuit. For example, look at V3 pin 3. You'll see a 3K to ground. But if you follow that wire up and to the right you'll see it connects to a 27K that connects to the OT secondary. The OT secondary has very low resistance so ignore that. That means the 27K is practically connected to ground, which puts it in parallel to the 3K at V3-3. Your meter will be measuring 3K||27K so the reading will be about 2.7K.

So, the take home on this is... When measuring resistors in circuit and the reading is lower than the resistor value there is probably another resistor in parallel even if it's not obvious. Looking at the schematic will usually give you an idea what your meter should measure. If the circuit is more complicated though, you'll have to disconnect one lead of the resistor.

[Platefire]

On V3B PI R35 the 47OK R reads 579K, R37 3K cathode resistor reads 3448 Ohms and R32 110K reads 123K.

I didn't realize it but was just checking but you can't buy a 3K or a 110K resistor anymore from most sources. I was thinking for R37 the 3K, I could install a metal oxide 3.3K(that measures 3.25K) and for the R32 to ground, install a 100K metal oxide. That would be installing common values now available and would be close to the values its already drifted to. That way it won't be drifting further out hopefully anytime soon. Good or bad idea?

Also got a metal oxide 470K for R35.

 

[PRR]

> you can't buy a 3K or a 110K resistor anymore

So make it 100k and 2700 (drop 10% all around).

This isn't government work.

[sluckey]

> you can't buy a 3K or a 110K resistor anymore

So make it 100k and 2700 (drop 10% all around).

If you decide to do this then also change R36 to 100K just to keep the cathodyne outputs balanced.

I probably have 3K and 110K resistors. I'll look tomorrow. Looks like you need 1/2 watt?

[Platefire]

I checked and found some metal oxide 2.7K that measures in the 2650 range. Got plenty of 100K's

[sluckey]

I probably have 3K and 110K resistors. I'll look tomorrow. Looks like you need 1/2 watt?

I checked my stock. Plenty of 3K and 110K but they are all way over spec.   

[bmccowan]

Time to shake and bake them, dip them in secret sauce, scrape them, paint them - whatever it takes.
This ebay seller has those values in 1 watt metal film.

https://www.ebay.com/itm/324683051749?hash=item4b989e32e5:g:DdUAAOSwJ2hgzXRu&amdata=enc%3AAQAIAAAA4K99NfT63TdFIG75dikOW14Uwl%2FVnDdTq9mDeBFLiMYOhebMtrCWG%2BGooM8CS8EJAAFllyV0PUJNtggeBxJbjT7Fv5PX39Zkldd1Zby6UTLM0Je6orYaoBZTsLYeumfryBuViE4M4YMqpvOY%2Bfr9uucj2Xr%2Fw6TlVEiv1GAlzzG82usv8I9QCRKsY5z30Pp2ftvkV7OVWkCv7tOftfwSv7zWbkcthB1eMYRGmIr%2F2jy3cNvCiuJRPAlcA%2BAxbPgypAIw2GCO1MXQI5rNAcDzdV%2F1TKJTC3HkGtAqI7kRhNPo%7Ctkp%3ABFBM2KKa96Ri

[Platefire]

Well thanks for looking sluckey. I got a bunch of CC resistors I picked up years ago but no 3K or 110K in mine either. Well I guess I forgot about e-bay and free shipping too. Thanks bmccowan

[Platefire]

Well I ordered them. Won't ever have to worry about running out of those values for this amp because for $2.99 they send10 Ea:>)

[Platefire]

sluckey----Got a question for you. It's regarding the "Loudness 500K, Tap 120K & 350K" or we would call "Master Volume".

Only thing this 4 term 500K pot has a component with multiple leads attached to it as shown on the schematic(attached for your convenience). This is something I left in place from the original circuit that I really don't understand its function. So if you could explain to me what this is doing, I truly would appreciate it.

What sort of Monster do we have here?

[sluckey]

It's a tone compensating network. Probably improves the bass sound at lower volume levels.

[Platefire]

Thanks sluckey for having look and letting me know. As I remember I started to rip all that out and just install a 1Meg Audio pot in its place but all the pots have long split end shafts to reach the face plate and for the existing knobs to fit, so I left it as it is. 

On the parts sheet is says K1 is "Tone Compensation" that was a special single component with multiple leads in the old tone stack that was removed.

On the parts sheet is says K2 is "Loudness Compensation" that is also a single special component with multiple leads that remains

 I'm thinking that possibly you could disconnect the special Loudness Compensation Component and just use the three top terms input/output/ground like a regular pot. Don't know what that would do to the sound that I'm use to? The loudness pot has 5 terms on it, 3 at the top, two at the bottom.

[sluckey]

Just remove K2 and you will have a standard 500K volume control.

[Platefire]

Been thinking about it and believe that's exactly what I'm going to do, cut the cord to K2. That's more of the design I had in mind way back then will finally be fulfilled. K2 is mostly Hi-Fi design, not guitar amp.

Also been thinking about that existing 3K NFB resistor in the place of the 27K shown on the schematic. I'm thinking of taking the standby switch and wiring and installing it as a 27K NFB on/off switch. I noticed that some vintage Fender 6L6 PP amps use the 27K NFB resistor. Off course I can test the 27K NFB operation before I put it all back together again.
Thanks for your help. Trying to do a good job on it

[Platefire]

Correction!! The NFB resistor I thought was 3K is actually 27K like it should be. It's just in parallel with the 3000 CC cathode resistor and measures 3.14K. A 3K and a 27K in parallel is 2700 Ohms. So considering drift, 3.14K seems about right. After all,the 27K color code red/blue/orange can be clearly seen + it's a metal oxide 1 watt, so I doubt its drifted much. It's the CC 3K that's done most of the drifting