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Thread: Low Power from Cathode Biased 2x6V6 Magnatone

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    Low Power from Cathode Biased 2x6V6 Magnatone

    Hey guys, how's it going around here? It's been awhile since I've been around. I've been playing music so much for the past few years I haven't spent much time working on amps. But I have an old Magnatone Troubador here that belongs to a good friend that I'm freshening up for him. It's a rather simple circuit, that uses a 6SJ7 pentode for the preamp, a 6N7 twin triode for a paraphrase style PI, and a pair of cathode biased 6V6's for power. When I got the amp it had the original cap can, plus a couple of 10uf individual were added to replace 2 of the original caps, but the main 20uf caps in the multi cap can were still being used. The power tubes were a mixed pair of old tubes that both tested weak. I found a new cap can at CE Distribution with 2 x 20Uf & a x 10uf sections, so I used that to replace all of the filter caps. I replaced all tubes with new, and gave it a general clean up, resoldered some questionable joints, cleaned up the tube sockets, jacks, etc. The amp soounds really good, but the power is really low, like around 4 - 5 watts output, and that's pushing it.

    You can view a schematic here, though it's hard to read, the resolution is real low: https://www.magnatoneamps.com/schema...one_M192-5.jpg

    Voltage readings are:
    B+ = 300
    Power tube plates = 299
    Screen grids = 298
    Kathode resistor (150 ohm) = 14.8

    Voltage readings at the output is abut 4.2vac at onset of distortion. Interesting, the voltage doesn't really change when I change the resistance of the load, from 4 to 8 to 16, the voltage remains about 4.2 volts. The speaker in the amp is an 8 ohm speaker, but it is not original, it's an old Ampro labeled speaker, must be out of an Ampro cabinet.

    What do you think? It looks like I have plenty of time to fiddle with this thing now, what with the Covid lockdown, I'm not going anywhere.

    Thanks for any help or advice you might have.

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    Supporting Member The Dude's Avatar
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    Here's a slightly better/tweaked schematic. Did you check or change the cathode bypass cap at the output tubes?

    Click image for larger version. 

Name:	LGJMQ20.jpg 
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    Senior Member hasserl's Avatar
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    Quote Originally Posted by The Dude View Post
    Here's a slightly better/tweaked schematic. Did you check or change the cathode bypass cap at the output tubes?

    Click image for larger version. 

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    Replaced with new 25uf 25 volt electrolytic.

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    Senior Member hasserl's Avatar
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    Also note: since the field coil speaker is gone, it's using a 1k5 dropping resistor, between the 2 20uf caps. Maybe I should upload my drawing of that part of the schematic

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    You could have a loss of gain before the power tubes. Do you have a scope? If so look at the power tube grids and see if you're clipping there. Are plate voltages good on your preamp and PI tubes? Possibly a bad plate resistor?

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    Senior Member hasserl's Avatar
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    I do have a scope. I'll check the power tube grids. Volts at the preamp & pi plates are high compared to the schematic you posted. That schematic does help. I can see the original power tube cathode resistor is 250 ohms, not 150. But that's not going to significantly change the power output. Still, the tubes are a little hot, so I might try changing that resistor. I'll see if I have one in my stash.

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    Don't confuse loudness with power. The amp isn't as loud as you expect, but that may have nothing to do with the power tubes. Dude is on the same wavelength I think.

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    Speaker efficiency?

    Justin

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    Actually, the volume isn't bad, it's the actual power measurements that's bothering me. The amp sounds fine to me, very nice actually.

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    As the the OT center tap is connected after the 1k5 dropping resistor, available supply current and power are strongly limited. As a consequence B+ may drop to 150V at full power (please measure). Such low B+ will not allow for more than maybe 5W output.

    I would connect the OT CT before the 1k5 resistor. If you want to reduce the increased B+, insert a dropping resistor of a few hundred Ohm between the two 20µ reservoir caps and take B+ from the second cap.

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    Last edited by Helmholtz; 03-18-2020 at 05:17 PM.
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    I'm sorry, I probably didn't describe the B+ and the filtering very well, or correctly. Here is a quick drawing I made of the current arrangement. This is how the amp came to, except with fresh caps & tubes. The voltages are with new caps/tubes. Mains voltage at my bench was 117 vac Click image for larger version. 

Name:	20200318_093729[1].jpg 
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    Looks like the rectified voltage at the 5Y3 cathode is considerably lower than in the schematic.

    - Is the 5Y3 good?
    - What is AC voltage at the HT winding?
    - What are heater voltages (for reference).
    - What is B+ at the OT CT at output clipping?

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    Quote Originally Posted by The Dude View Post
    You could have a loss of gain before the power tubes. Do you have a scope? If so look at the power tube grids and see if you're clipping there. Are plate voltages good on your preamp and PI tubes? Possibly a bad plate resistor?
    Signal at the power tube grids is clean and smooth when the output starts to clip. Voltages at the girds at 6.46vac at one, and 4.84vac at the other. Slightly asymmetrical, but I don't think that's so unusual with a paraphrase PI is it?

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    Quote Originally Posted by Helmholtz View Post
    Looks like the rectified voltage at the 5Y3 cathode is considerably lower than in the schematic.

    - Is the 5Y3 good?
    - What is AC voltage at the HT winding?
    - What are heater voltages (for reference).
    - What is B+ at the OT CT at output clipping?
    Tested with new 5Y3 and existing rectifier tube, no difference.
    A/C voltage at the rectifier socket are 311/313
    HEater voltage is 6.4vac at the pilot lamp
    B+ voltage at the OT center tap at the start of clipping are essentially the same as at idle, maybe 1 volt higher. From that point on the voltage goes up as clipping increases.

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    Someone please correct me if I'm wrong, but with a cathode voltage of 15VDC, I think you should be able to get around 10VAC at grids before clipping?

    Quote Originally Posted by hasserl View Post
    Signal at the power tube grids is clean and smooth when the output starts to clip. Voltages at the girds at 6.46vac at one, and 4.84vac at the other.

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    I think you should be able to get around 10VAC
    Yes, but we don't know how much grid drive the PI is able to provide. The fact that the output clips before the grid signal means that the clipping is not caused by the PI or by grid conduction.

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    B+ voltage at the OT center tap at the start of clipping are essentially the same as at idle, maybe 1 volt higher. From that point on the voltage goes up as clipping increases.
    That's very unusual. In a class AB amp current demand increases with output causing B+ to drop.
    There is a possibility that the amp actually runs in class A, which is very inefficient and not really desirable (from a technical POV) for PP stages. If the amp runs in class A will be determined by the idle currents and OT primary impedance (being very high). In class A current demand essentially stays constant with output.

    What is known about the OT, is it original? Does that 150R cathode resistor look original?

    Still I doubt that B+ could increase as clipping increases.

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    @ hasserl: Sorry, why a thumbs down?

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    Quote Originally Posted by Helmholtz View Post
    @ hasserl: Sorry, why a thumbs down?
    I'm sorry, I hit the wrong button! Bad eyesight. My apologies. I better go back and see if I gave anyone else a thumbs down, definitely not intended! I appreciate each and every response. Sorry guys.

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    Do you have a chance to try a different OT with a primary impedance of 8k?

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    Quote Originally Posted by Helmholtz View Post
    That's very unusual. In a class AB amp current demand increases with output causing B+ to drop.
    There is a possibility that the amp actually runs in class A, which is very inefficient and not really desirable (from a technical POV) for PP stages. If the amp runs in class A will be determined by the idle currents and OT primary impedance (being very high). In class A current demand essentially stays constant with output.

    What is known about the OT, is it original? Does that 150R cathode resistor look original?

    Still I doubt that B+ could increase as clipping increases.
    I believe the OT to be original. The cathode resistor is not original, and apparently is not the same value as stock (according to the schematic the original value is 250 ohms, modified resistor is 150 ohms).

    B+ definitely increases as clipping occurs. I assume current flow through tubes decreases as clipping occurs, causing the increase in B+.

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    Quote Originally Posted by Helmholtz View Post
    Do you have a chance to try a different OT with a primary impedance of 8k?
    If I had one on hand I could try that. I don't.

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    Have you tried it with a 250 ohm instead of the 150 ?

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    Quote Originally Posted by g1 View Post
    Have you tried it with a 250 ohm instead of the 150 ?
    NO, I haven't yet. I don't think I have one in stock, and I'm not looking forward to going to a local store to try to purchase one, just trying to avoid going anywhere with human interaction right now. I need to place an order for some other components since I'm running low of commonly used items, I'll try to get a resistor with that order.

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    Anything you can stick in series with that one to test?

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    B+ definitely increases as clipping occurs. I assume current flow through tubes decreases as clipping occurs, causing the increase in B+
    As the total supply current drawn by power stage must flow through the cathode resistor, cathode voltage is an indicator of actual supply current.

    If B+ increases with clipping, cathode voltage should decrease.

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    Quote Originally Posted by Helmholtz View Post
    As the total supply current drawn by power stage must flow through the cathode resistor, cathode voltage is an indicator of actual supply current.

    If B+ increases with clipping, cathode voltage should decrease.
    It does. See next post

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    Senior Member hasserl's Avatar
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    Looky looky what I found hidden in the bottom of a drawer:
    Click image for larger version. 

Name:	20200320_170218[1].jpg 
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ID:	57465 (ETA: well that's not a very clear picture. It is a 250 ohm 5 watt resistor)

    So, with the new cathode resistor I rechecked all voltages (note mains are slightly higher today)

    A/C HT at the rectifier = 315/317
    B+ = 332
    C+ = 326
    Power tube plates = 325
    Screens = 323
    Cathode = 20.3
    A/C at 4 ohm load resistor at start of clipping = 4.43
    B+ at start of clipping = 332

    As I increase the clipping beyond that point the B+ immediately starts to climb, up to a high point of 342. At the same time K voltage decreases, to a low point of 17.8 (at the same time as max B+).

    Could it just be that the OT is too small to produce any more output, that it's just saturating, causing the clipping in the output?

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    Have you scoped the ripple on B+ at full power and at clipping?

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    As I increase the clipping beyond that point the B+ immediately starts to climb, up to a high point of 342. At the same time K voltage decreases, to a low point of 17.8 (at the same time as max B+).
    Ok, so measurements seem to be consistent.

    I think it's like this: The power stage operates in full class A (very unusual with PP amps). In cathode biased class A amps cathode voltage essentially stays constant in the linear range. As clipping occurs and the output signal gets squared, tube dissipation reduces because the tubes are now essentially switching between 2 states of low dissipation. Consequently B+ rises.

    The class A operation is due to the high idle current (is the 20.3V with a 250R cathode resistor?) and most probably an unusually high OT primary impedance (>>10k?, actual primary impedance can be calculated from voltage measurements at the disconnected OT). The reason for the high (reflected) primary impedance could be that the OT was rated for a load impedance below 4 Ohm. A high load impedance would also explain the high voltage gain of the power tubes showing in the low grid drive required for output clipping.

    Output clipping is no sign of core saturation. Rather saturation typically shows as something like crossover distortion at zero crossings without limiting voltage peaks.

    Could you post a scope picture of the clipped output?

    What is screen voltage at full output?
    I would also check grid coupling caps for leakage.

    It should be possible to get around 12W of clean power from this amp with an appropriate OT.

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    Screen voltage drops about 7 volts from idle to max clean output.

    Screen shot of scope with clipped output. I didn't know how clipped you wanted it, so I just grabbed this shot with both positive & negative swings clipped. (wtf, picture keeps coming out sideways)
    Click image for larger version. 

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    Thanks.
    Picture shows almost symmetrical clipping. No signs of OT saturation.
    Scope needs adjustment of trace rotation.
    Please always post scope settings with pictures (vertical sensitivity and time base coefficient).

    Your cathode voltage with 250R is 25% higher than specified in the schematic. That could be caused by a leaky coupling cap. To check for leakage, disconnect one end of the cap(s) and watch for changes in cathode voltage.

    Otherwise no new ideas. Is there any spec regarding amp output power?

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    Quote Originally Posted by Helmholtz View Post
    Thanks.
    Picture shows almost symmetrical clipping. No signs of OT saturation.
    Scope needs adjustment of trace rotation.
    Please always post scope settings with pictures (vertical sensitivity and time base coefficient).

    Your cathode voltage with 250R is 25% higher than specified in the schematic. That could be caused by a leaky coupling cap. To check for leakage, disconnect one end of the cap(s) and watch for changes in cathode voltage.

    Otherwise no new ideas. Is there any spec regarding amp output power?
    None that I can find, there doesn't seem to be any old ads or marketing or product literature that I can find. I'm just considering the tubes and the voltages involved, and power just seems low. Even a 5E3 Delux is good for at least 8 - 12 watts.

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    Even a 5E3 Delux is good for at least 8 - 12 watts.
    The primary impedance of a 5E3 OT is around 8k/8R. Your measurements indicate that your OT has significantly higher primary impedance. To find out you need to determine the voltage transfer ratio.

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    Quick check would be to run it into a 2 ohm load if you have one. Power out should increase.

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