Hi Fi to Champ conversion questions: Voltages too high?

[Regis]

Hi Guys,

I rebuilt an old 1958 Voice of Music 8810 into a Champ and while it was a success, I have a few questions and some issues to resolve.

The amp is a combination of a couple of schematics from KOC's TUT3. It's basically a 5F1 that is voiced to have no negative feedback. I left out the tone controls, we don't need no stinkin' tone controls. He had a master volume in the schematic but I left that out too. I've cobbled together a schematic of what it is and posted it here. It sounds really good. It has incredible scorching overdrive and not much else, a one trick pony. That's ok, I like it like that. I sounds awesome driving an Eminence Private Jack. I expect it would sound good through a Greenback too. It's a lot louder than I would have expected a Champ to sound, even running it through a 12 in speaker. It sure seems like it's putting out more than 5 or 6 watts. Also I don't have a lot of experience with Tweed Champs but it doesn't really sound like a Champ, in fact it doesn't really sound like a cathode biased amp.

It uses a 6x4 rather than a 5U4 because that's what was in the VOM. I used a 20/20/20 450 volt filter can from Ted Weber. I have several versions of Champ schematics that show different values of filter caps but KOC used the 20/20/20 so that's what I used.

I get some hum but not too bad. I know Champs are famous for that. I measured both sides of the 150 ohm heater balance pot and adjusted it for equal voltage on each leg.

The amp originally came with a 6V6, and my plan was to wire the power tube socket to accommodate a 6V6, a 6L6, or an EL34, which I did. Sort of a poor mans Univalve. I'm just able to shoehorn an EL34 into it, it's a tight fit. I have to unscrew the top half of the chassis to squeeze it in. It runs pretty hot too, I'll probably figure out a way to put a fan on it.

My biggest issue is that the voltages are too high to use a 6V6. I fried one the first day I fired it up, which got me to checking the voltages. I know I should have checked that before anything but whatever. I put another in it long enough to measure voltages and see that it was redplating.

I looked over the datasheets for the 6L6 and the EL34 and if I read them correctly the voltages in the amp are ok for them for a class A amp, but too high for a 6V6.

I put my bias checker on it to get the bias. If I understand it correctly in a cathode biased class A amp, maximum bias should be plate voltage divided by max dissipation of the tube? In other words for a 6L6GC in this amp it would be 30/418 ? The 6L6 and EL34 are within specs there if so. I'm using a 470 ohm cathode resistor. My understanding is that the 70% rule regarding bias doesn't apply to class A amps. Please correct me if I'm wrong.

Here is what I measured. The Max bias data is what I get when using the bias formula with the amp voltages:

[FONT="Courier New"]6L6:		Max	Actual
------------------------------
Pin3 Plate:	500	418
Pin 4 Screen:   450     411
Bias		71ma	66.7ma

EL34		Max	Actual
------------------------------
Pin3 Plate:	800	403
Pin 4 Screen    425     399
Bias		62ma    61ma

6V6		Max	Actual
------------------------------
Pin3 Plate:	350	419
Pin 4 Screen    315     411
Bias            28ma    61ma

12AX7		Max	Actual
-------------------------------
Plate           300	252[/FONT]

So... questions:

How do I get the voltages down to accommodate the 6V6?
To do so do I add more or different resistance after the rectifier?
If I do, will there be a significant volume (power) and tonal change with the EL34 and 6L6?
If I do, is it possible I could make it switchable to keep the voltages for the EL34 and 6L6?
What value of fuse should I use? I have a 1 amp in there now, I was wondering if this is too small.
Is there any way I can measure how much output wattage it’s generating with only a multimeter?

If I have to I’ll just leave it the way it is and use 6L6’s and EL34’s, but it would be nice to be able to use a 6V6 too.

Thanks,
Regis

[Marc]

VOM champ

Regis,
The voltages you quoted didn't seem too far over the top for a healthy 6V6....
Did this amp originally have the power tube cathode going to the OT? I had one of these and that was what cathode configuration it had. If it did have you tried it that way instead of with a resistor?

It also had interesting tone controls that were a kind of negative feedback control. There's bound to be a corrct name for them but I don't know it. Harp players tend to like these. The amp I used to have now belongs to a guitar slinger in Texas.

Marc

[bob p]

Hey Regis -

I would agree that those voltages are pretty high, way beyond the design maximum rating for a 6V6. But then, SF Champs and Deluxe Reverbs have always run at pretty high voltages around 400 - 410 VDC. Voltages like those can tend to separate the men from the boys as far as tubes go. Some 6V6 can take it and some can't. To make sure that your tubes can survive, there are a few things to think about.

First, I'm not sure what your load impedance is. As you may know, plate load impedance differs pretty dramatically for the 6V6, 6L6 and EL34. Oh -- I just noticed that you said that the amp originally came with a 6V6. If you stiffened up the filtration in your B+ supply, that could have helped to nudge your voltages upwards.

Another thing to consider is that load resistance will change with plate voltage. This is a number that's a little harder to get a handle on, becuase the 6V6 tube data sheets don't list operating voltages as high as yours, which makes it impossible to interpolate between two known values to estimate your operating conditions. Maybe someone with modelling software might have a better handle on this aspect of things. In the big scheme of things, this consideration may not be that significant as I had originally thought because your OT was orignally designed for a 6V6.

Regarding how many volts are really on your plates, its important to remember that the voltage on your tube is really the plate voltage minus the cathode voltage. I didn't see the cathode voltages in your spec table. Knowing them would be really helpful, first to determine your real plate voltages, and second to calculate the total dissipation for the tube.

One thing that I've noticed about 6V6 is that the weak ones that tend to fail in high voltage deployments like yours seem to suffer from screen failure. When you saw your original tube red-plating, are you sure it was the plates and not the screens that were glowing red? Screen failure is a pretty common problem in 6V6 that get run at excessively high voltages. Keep an eye on your screen dissipation in watts and make sure it doesn't exceed the tube's rated limits. Change the screen resistor to keep the screen dissipation under control. In my experience (other people might have different experience), good new production 6V6 should have no trouble handling plate voltages like yours if the screen dissipation is kept under control. As far as new produciton tubes go, I think that both the EH 6V6 and the JJ 6V6 are tubes that can live up to the task if you protect the screens.

On the subject of bias, for a circuit like yours I prefer to bias by looking at the cathode voltage and the cathode resistor value, and calculating cathode resistor current. Granted, that gives you a total of plate dissipation + screen dissipation instead of directly measuring plate dissipation. The advantage to doing it this way, at least as I see it, is that by including screen dissipation in your figure, you will bias the amp a little colder. A conservative approach like this puts you on the conservative side of the correct bias range, and helps to extend tube life.

If you really want to get a direct handle on the B+ voltages, its easy enough to create a switchable B+ for the times that you run 6V6 in the amp. For a SE amp like yours that's cathode biased, one method of B+ reduction is to connect a zener diode (or a string of them) to the center tap of your high voltage secondary, and then connect the other end to a switch that goes to chassis ground. (be sure to orient the zener(s) with the correct orientation) That will lower your B+. For a Class A amp, you could theoretically use a big power resistor in place of the Zener, though I've never done that. IMHO these approaches are better than adding a large value resistor to the filtering network. Maybe someone else will have a better idea.

Hth.

Bob

[bob p]

forgot to mention - some people notice a signficant difference in tone of the 6V6 when they're run at the "tweed" voltages of about 340-360 VDC, compared to the much higher "SF" voltages. depending upon your objectives, you might want to just drop the voltages marginally to protect the tubes, or you might want to try to drop them all of the way to tweed levels for tonal purposes.

dropping the votlages that far using the zener trick would probably require a string of them in series. another option is to put a voltage divider network of power resistors into your filter network, so that you could have taps at the desired votlages. doing that converts our amp from a Cap input (Pi) filter into a Resistance input filter. that approach may or may not sound good (I've never done it). Maybe someone else migh have a better idea.

[stokes]

That 61ma's on the 6v6 is way too high,that is the reason for the tube failure,not the 400+ volts on the plate.At those volts you want to be closer to 30ma's.Here is a link to a bias calculator that is very handy-www.webervst.com/tubes/calcbias.htm.As for dropping the volts,if that is what you want to do,the lower "tweed" volts will give you a "browner" more easily broken up tone.You can get a "reverse oriented" zener from Mouser part #NTE5278AK,it is a 56v/50watt zener,you just drill a hole in the chassis,screw it in and connect the CT,much easier than using a string of diodes.I got a 47v drop with this diode.Personally,I like the somewhat higher volts in mine,along with a bit more filtering on the main and screen,I used 40uf's here,and an 8 or 10uf max on the preamp,20uf is too much IMO.I know someone will point out that the spec sheet for the 6X4 says 10uf is recomended,but I have used 100uf with various rectifiers that say 20uf is the max and the tubes have lasted for years.In fact my first 5E3 clone used a 6X4 and I used an 80uf main and 40uf on the screen and it is still going fine.A simple test is to look at the rectifier when the standby is switched on and if the rectifier doesnt arc it can handle the extra filtering okay,as once the filter is charged up it wont put any more strain on the tube.I like the extra headroom and tighter response with the higher filtering,and the amp will still break up nice when pushed.

[J Martin]

A choke input power supply would also lower your voltage.

[Regis]

Marc, yes, it had the cathode attached to the OT. The amp sounded like ass to me with the original configuration so I rebuilt it into a Champ.

Bob, as always a detailed explanation, thanks. I didn't know that about the plate voltage-cathode voltage thing, I'll measure that and post it. No, I'm not sure about the screens/plates redplating, but I thought it was the plates. It wasn't that much glow, but it was there.

Stokes, always glad to have you chime in. I knew the bias had to be lowered, do I change the cathode resistor to change that? How do I figure out what value? Could I make it switchable, or would that even be necessary?

Thanks for the zener diode idea, guys, that would be an easy fix, could I make that switchable? That way it would give me the louder 6L6 tone and the traditional tweed 6V6 tone, and solve my 6V6 problem.

I have a spare 20uf section on that cap can, I could parallel that with the 20uf section I am using for the preamp tube.

This amp doesn't have a standby, do you think it would be a good idea to add one? KOC sez Champs don't need 'em.

J Martin, this amp is more crowded than a Japanese subway car at rush hour, there is no place to put a choke. The OT is actually inside the chassis, underneath the PT.

Oh, and I'm not sure about how healthy the 6V6's I'm using are... they are all vintage tubes: two VOM made by Westinghouse, a Silvertone, and an Emerson. The one I fried was an RCA.

Which brand of the new 6V6's do Y'all recommend?
It also occurs to me to replace the 6X4, as it is 50 years old as well.

Anything else I need to check and post?

Thanks for the responses.

Regis

[stokes]

To change the bias on the 6V6 you would adjust the cathode resistor,and yes,you can and should make it switcable,as you will need a different value to bias a 6L6 and 6V6.If you check the bias calculator link I gave you you can get your target currents for the various tubes.You will have to experiment to see what value the tube likes.A higher value resistor will lower your current.The zener can be switched in and out as well.The easiest way to figure the plate voltage is to either subtract the cathode voltage or just measure from the plate to cathode,put your black lead on the cathode and the red lead on the plate.I wouldnt parallel the extra 20uf with your preamp,that will give you 40uf on the preamp,way too much,it will hurt your tone.I would put a seperate 20uf in series to bring it down to 10uf,but you cant series a can cap with itself,you would need a seperate cap to put in series with the can section,but you could parallel that 20uf with the 20uf on the main to bring it up to 40uf.As for a standby switch,it is debatable,many say there wont be a problem with cathode stripping in a low power guitar amp without one.I feel that if you are using a delayed startup rectifier such as a 5V4,you dont need one,otherwise I would use one.Not sure offhand if the 6X4 is a slow startup or not,sorry.Looking at the schem you posted I would also change those 470n bypass caps to 22uf or so.If those vintage 6V6's are in good shape they are likely better than any new production tubes made today,too bad the RCA got fried,but I would think it didnt fry because it was a bad tube,it was more likely due to way too much current.Did you check all the 6V6's for current draw?Another thing I noticed on the schem that I would change.That 1k resistor after the first filter cap.If your plate voltage,after you subtract the cathode voltage drops below the screen volts,which I strongly suspect it will,I would try a 5k in place of the 1k to bring the screen volts down below the plate.But if you havent checked the other 6V6's yet,I would check them first to be sure that RCA wasnt bad to begin with,the Champ uses a 470ohm on the cathode and the volts and current you posted indicate you would need a much higher value,so as an after thought,it could be a bad tube,and a good tube might fall into a colder bias condition with the 470ohm cathode resistor.

[bob p]

To change the bias on the 6V6 you would adjust the cathode resistor, ... You will have to experiment to see what value the tube likes.A higher value resistor will lower your current.

Regis, if this stuff isn't clear to you yet, think of it as a simple Ohm's Law problem for the cathode resistor. Remembering that V=IR, if you keep voltage constant (in your mind), increasing the value of R will result in a lower value for I. In practice V doesn't really remain constant, and you'll have to repeat your measurements and approach the problem with successive approximations to dial in the correct cathode resistance. That's as good a reason as any to try the adjustable resistor trick that @stokes mentioned.

As fas as new vs. old tubes go, it might be worth considering the purchase of new production 6V6 as guinea pigs until you get the bias problem solved -- if you make a mistake, its better to blow up an inexpensive new production JJ than an NOS RCA.

[stokes]

When I ran out of the adj.wire wound resistors,I ordered a 500ohm 25watt rheostat from Weber,think it was less than $5.I have that mounted on a piece of wood with 2 alligator clip leads and a 50uf 100v cap across it,you can just dial in the needed resistance and then get a resistor as close as possible,it doesnt have to be exact as the circuit tends to balance itself when the signal is applied.To further simplify bobs simplification of V=IR,as you raise the current your plate volts will go down,lower the current and the plate volts go up.Its a balancing act sort of.Reading your post again,concerning the hum level a good trick to reduce the hum drastically is to lift the ground on your hum balance pot and connect it to your power tube cathode,this puts your heaters ground reference 20+v's above your signal ground and keeps the 60hz out of the signal.It is probably a good idea to use a new 6V6 for testing,but as long as you dont try to play the amp with the current at 61ma's like you did before you shouldnt have a problem blowing the tube again,if the tube were to blow while idling,it is most likely a bad tube anyway.Never try to play it till you are sure the idle current is "safe".

[Regis]

Guys, thanks for all the info. It's been an education, and some of it is still sinking in.

I have more questions, but I am going to wait until I get a new 6V6 this week to experiment with. I'll do those measurements and get back here with the results.

One thing I want to say is that I want to keep the tone and volume I have now with the 6L6 and EL34 if possible, while still being able somehow to run a 6V6 if I can. That's why I asked about making things switchable.

Thanks,

Regis

[Regis]

Ok, I checked the voltages again, and added another 470ohm resistor in series with the cathode resistor already there.

Before adding the resistor:
Cathode = 29v
Plate = 409v
Screen = 411v
Plate minus cathode = 380v

After adding the resistor:
Cathode = 35v
Plate = 440v
Plate minus cathode = 405v
Forgot to measure the screen voltage.

I bought an EH 6V6 and installed it, and after adding the resistor I got 36.8 cathode bias current.

If this is right:
Divide cathode voltage by cathode resistance:
35/940 = .037

Multiply Plate Minus Cathode by .037:
405*.037 = 14.9 watts dissipation

The EH is a 14 watt tube, (specs below) so it more or less is within spec. I could put a little larger resistor in there to get the wattage down a little bit.

Question: The specs on the EH 6V6 say it can take up to 80ma cathode current, does that mean I could put it in without trying to lower the cathode current with the extra resistor? With a 470 ohm bias resistor I get about 61ma of current.

Of course, a 12 watt 6V6 would burn up. Or I could keep adding resistance to get it to use a 12 watt tube.

Now having said all that, the tone is dramatically changed. It isn't as loud and doesn't put out as much power. The distortion is a lot smoother and not as raunchy. It probably now sounds more like a real Champ. If I'm gonna keep this I have to figure out a way to shoehorn a switch into the chassis to be able to go back and forth.

I'm going to order one of those Zeners from Mouser and try that as well.

I was reading up on the Univalve and they use a switch to lower the voltage to be able to use a 6V6, I wonder how they do that?

[opacheco]

Hi Guys,
................The amp is a combination of a couple of schematics from KOC's TUT3. It's basically a 5F1 ...............
Regis

What mean that??

Opacheco.

[Jag]

KOC's TUT3 = Kevin O'Connor's Utlimate Tone Vol 3. A very useful and informative set of volumes on amp building and modification.

5F1 = Fender Tweed Champ from the late '50's. There were a number of revisions to the Tweed Champ circuit and many consider the 5F1 version to be the best sounding.

[opacheco]

KOC's TUT3 = Kevin O'Connor's Utlimate Tone Vol 3. A very useful and informative set of volumes on amp building and modification.

5F1 = Fender Tweed Champ from the late '50's. There were a number of revisions to the Tweed Champ circuit and many consider the 5F1 version to be the best sounding.

Jag
Thanks,