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Post subject: High plate voltage in my 5E3 hybrid
Posted: Tue Mar 20, 2012 7:23 pm
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I would like to pick up from my last topic concerning my hybrid 5E3. First, thank you to everyone who chimed in, your advice was well read and appreciated.

The amp is running and it sounds pretty good. However, I have a concern about the plate voltage on V1-pin 6. It is right about 300 Vdc +/- a few. I replaced the plate resistor just to be sure it was working correctly. Pin 1 195 volts, so I am at a loss to explain why this one pin is so high. Here are the voltages:
P1 P2 P3 P4+5 P6 P7 P8 P9
V1: 186, 100Mv 1.7 3.28Vac 308 100Mv 4.64 3.28
V2: 198 4.4Mv 1.7 3.28Vac 251 22.3 60.8 3.28
V3: n/a 3.28 397 345 -22 347 3.28 45Mv
V4 n/a 3.28 395 344 -22 345 3.28 46Mv
V5 402Vdc 344Vac 344Vac 402Vdc

I am using 1R current sense resistors across pin 8 for bias measurements.

Resistance of each side of the OT is about 220R, CT to V3/P3 is -9.5, and V4/P3 is -10. I have not seen any red plating either. Thanks in advance for your help.

Robert


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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Tue Mar 20, 2012 8:06 pm
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I don't understand using 1 ohm resistor on the Cathode of the output tubes on a Cathode-biased amp, that doesn't work. What do you mean by: "across pin 8"? In a fixed bias amp, the 1 ohm resistor would be connected from the Cathode of both output tubes (pin 8 ) to ground, not from pin 8 of one tube to pin 8 of the other tube.

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Tue Mar 20, 2012 9:30 pm
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Shimmy, I think Bigdaddy is just using the 1ohm to translate the current. If I remember correctly, his amp is fixed bias. Robert, the 308vdc is tolerable on the second half of the 12AX7( or am I wrong about that tube). If that doesn't suit your purposes, I'd backtrack down the B+ supply line to the power filter, and adjust it accordingly. IMHO The higher plate voltage will serve to keep that second stage clean. But, I'm not totally familiar with your circuit design. BTW nice synopsis of the readings! It would help if we all got into that habit ! Art

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 6:09 am
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Fixed bias? Where does the negative bias supply for the output tubes grids come from? So there is no cap/resistor on pin 8 to ground of each output tube? Not quite a 5E3.

Anyway, both plates of V1 should have the same voltage (pin 1 and pin 6), if they each have the same circuit (ie, each have 100K Plate resistors and same Cathode circuits). With the voltage higher on one side, that tells me that the side of the tube with the higher voltage is not conducting. So if both Plate resistors are close in resistance, look at the Cathode components for each (res/cap on pins 3 and 8 ). The voltage for both pins 1 and 6 come from the same supply (normally that is), no point in tracing back the supply, it's there as evidenced by the 300 volts on one Plate. Could even be a bad tube (half).

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 11:53 am
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Thank you for the replies. The 1R resistor on pin 8 of the power tubes goes directly to ground on each tube as a current sense resistor. I have tried two different tubes in the V1 position but there is no change in the voltage on pin 6.

I set this up with a split cathode arrangement; 22uf/1.5K on pin 3, and .68uf/2.2K on pin 8. I used a mod from Mission Amps which uses a dual 1M pot for the tone control and summing resistors on the wipers of the volume pots to allow for mixing of the volume controls ala Marshall. I use one input jack with a two conductor shielded cable, grounded to the pot bus, and 68K's to pins 2 and 7.

I was looking for suggestions on what to check regarding the high voltage as I know they should be the same. i will try a couple more 12AX7's and if that doesn't help I could try changing the bypass cap going to pin 9. I just checked and it is grounded correctly. Thanks for the input.

Bigampdaddy


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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 3:52 pm
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Ah! That makes more sense.

The circuits are not the same then. If you have different values of cap and res on each Cathode of V1, then the voltage will be different for each Plate. Check for a voltage drop across the Plate resistors to see the current through each tube at idle. If little or no voltage drop across the Plate resistor of the side with the higher voltage, check the Cathode components for that side. Obviously, a 22 uF is a long way from .68 uF, and coupled with different res values, I would expect a difference in the Plate voltage between the two halves.

What you have might be perfectly normal for the component values that you have chosen. Any rhyme or reason for the choice of values?

P.S.
Using 5E3 in the description of your amp seems way off. What part is still 5E3? :lol:

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 5:42 pm
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Yeah, I understand the rhyme. I've experimented extensively, with my Bassman bass channel. Those figures are typical for a Marshall set-up. It pretty much matters what sound you are looking for out of the pre amp. I'm not finished tinkering with the Bassman. Went a little overboard with the second gain stage. It's just bullying the last stage(it's basically an AA864, with too much hot sauce). Oh Well ! One project at a time. Art

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 7:09 pm
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I think that I understand now, your earlier post indicated that you thought the voltage was OK, which I dismissed, thinking 5E3 preamp where both Plates would have equal voltage. Sorry about that.

So, we have an amp that sounds great, and really has no problem? Is that right?

Cool!

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 7:30 pm
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Shimmy, not sure if all is fine, with Bigdaddy. But yeah, his amp is personalized. Haven't seen his schem, so I'm kind of guessing. Your input always gets us to look straight ahead! Not being an electronics engineer, but a musician with a little tube amp experience, that affords me the luxury of some fun. I better check to see if that fire extinguisher is charged! Art

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 8:05 pm
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shimmilou wrote:
Ah! That makes more sense.

The circuits are not the same then. If you have different values of cap and res on each Cathode of V1, then the voltage will be different for each Plate. Check for a voltage drop across the Plate resistors to see the current through each tube at idle. If little or no voltage drop across the Plate resistor of the side with the higher voltage, check the Cathode components for that side. Obviously, a 22 uF is a long way from .68 uF, and coupled with different res values, I would expect a difference in the Plate voltage between the two halves.

What you have might be perfectly normal for the component values that you have chosen. Any rhyme or reason for the choice of values?

I wanted to see how it would sound with the split cathode and the .68/2.7K Rk/Ck combo.

P.S.
Using 5E3 in the description of your amp seems way off. What part is still 5E3? Well let's see; the cabinet and the chassis are about the only things. :) :lol:


Shimmi, thanks to your last post I decided to disconnect the split cathode and make it a shared cathode as God intended. "Thou shalt not make a 5E3 circuit with a split cathode". Or something to that effect. :lol: Anyway, now the voltages on pin's 1 and 6 are right at 200 + or - a couple. I set the bias at 36mV and those old Tung-Sol's are very well matched. Only off by two millivolts from each other. They came from a '50's Hi-Fi I got from Ebay for about $40.00. The tubes are worth more than that. The plate voltages on them are about 402. When I can afford it I will get a GZ34 to replace the 5U4GB to see if they will drop a bit more.

A big thank you to everyone for your help and suggestions. I anyone wants the mod to split the volume controls just pm me and I will send it to you.

Bigampdaddy

It is 11:00 pm here in Florida so I can't crank it up at all. My neighbors do not share our appreciation for cranked amp tone. I don't know why? :)


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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Wed Mar 21, 2012 11:42 pm
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Way to go Man! Glad it's all cool. Yeah, Shimmy gets the real deal done! Art

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Post subject: Re: High plate voltage in my 5E3 hybrid
Posted: Thu Mar 22, 2012 11:33 am
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" I will get a GZ34 to replace the 5U4GB to see if they will drop a bit more. "

Very interesting amp. Let me suggest that the GZ34 may not drop the B+ at all. Yes, it's PIV is 50 volts less (1500 versus the 5U4GB 1550). BUT, the GZ34 has much less voltage sag than the 5U4GB (15 volts versus 50 volts). So, when the amp is in use, my bet is the RMS DC voltage is going to actually be a bit higher.

Like Shimmy, I'm having a hard time getting my head around the bias topology of this amp. You say it's based on the 5E3 circuit. Which is Class "A," cathode biased. But, then you say that you read the voltage drop across the cathode-to-ground with a precise 1-ohm resistor. If you have a cathode-tied resistor like the 5E3 (250-ohm, 5 watt), the only way to read the 1-ohm resistor would be to break the 250-ohm resistor's contact with ground and add the 1-ohm resistor in series.

But more importantly, you can't change the settings without changing that 250-ohm resistor. I've seen ppl replace the fixed 5 watt resistor with a 20 watt rheostat (big pot) and use that to alter the idle current. Is this is what is in your amp now?

Or did you remove the cathode-tied resistor and add a separate bias power supply (like a 6G3 circuit)? The reason this maybe important is the 5E3 is biased to run the tubes Class "A." And the 6G3 is setup to run the 6V6GT is Class AB1. There is a BIG difference in plate voltage and idle plate dissipation with the two topologies.

You CANNOT run 6V6GT in Class "A" at 402 VDC on the plates. And expect those tubes to last very long. 300-350 VDC with quality 6V6GT, yes. On the other hand, if you are running a bias power supply (putting negative voltage onto the grid--- like around a steady -25VDC--- AND the cathode of the 6V6GT are tied straight to ground (no cathode-tide resistor with a bypass cap, then 402VDC maybe fine. As long as you can adjust the idle to around 8 watts idle dissipation per tube and run the tubes in Class AB1... you'll be ok.

Anyhoo... do you happen to have a schematic of your amp. Or if you look at the two in the links below... which one "looks" more like yours. Esp around the 6V6GT's.

Mucho!

http://www.webphix.com/schematic%20heav ... _schem.pdf

http://www.webphix.com/schematic%20heav ... _schem.pdf


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