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Help needed, very confused.

45.132.115.88

Posted on December 29, 2020 at 11:02:36
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019


Power supply is a 250-0-250 81 mA transformer with a 5AR4 rectifier. B+ voltage is 250V.

At this point I must be doing something wrong. I thought that I had a very basic understanding of Ohm's law. Apparently not.

I am still working on a 27-6SF5-45 guitar amplifier. I will try to attach a schematic of the power supply and circuit.
The voltage measurements for each tube are written on the schematic.

I do not understand the measurements on the 45. 194 V on the plate, 1.5 V grid and 2.9 mV cathode. Using 200v (close to 194) on the chart, 45 mA crosses -30V nicely. To calculate a bias resistor isn't 30 divided by 0.045 A to get 670 ohms (I used a 750 ohm resistor as it is close).

 

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RE: Help needed, very confused., posted on December 29, 2020 at 11:16:08
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
You don't have 194 volts plate. You have the voltage at the plate minus the voltage at the cathode. It's the voltage across the tube that matters.

I have a 300b with 465 volts present at the plate. It is cathode biased with a resistor from the cathode to ground. There is 80 volts at the top of the cathode resistor. I have 385 volts "plate to cathode" and that is what I look at when I look at the plate curves. Not 465 volts but 385 volts.

Your cathode resistor is so big that it is cutting off the current through the tube.

BTW the grid resistor on the second stage goes before the grid stop resistor not after it.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Help needed, very confused., posted on December 29, 2020 at 13:15:15
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019
Thank you for the help. I clearly do not understand how the grid lines work. How do I arrive at a bias resistor value?

With 200 V at the plate how to I get to 30V on the cathode? This would give 170 V across the tube?

The 170 line crosses the 30V curve at 25 mA.

 

RE: Help needed, very confused., posted on December 29, 2020 at 14:02:49
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
There must be something else wrong.

For a 30 volt cathode bias, with 170 volts across the tube and 25ma of plate current, you would need a 1200 ohm cathode resistor.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: Help needed, very confused., posted on December 29, 2020 at 14:32:12
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019



As you know there isn't much in a SET circuit. All of the solders and grounds are intact.

The out put transformer is a new Classic Tone, wound like a beefed up Fender Princeton.

I don't have a large enough 1200ohm resistor. I do have a 1500 ohm 15 w resistor (should be close enough?)

Any change the filament heater could do this? The cathode voltage seems low (fillamets should be 2.5 volts).

 

RE: Help needed, very confused., posted on December 29, 2020 at 15:13:40
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
Can you post a schematic of the filament supply for the 45 and how you are connecting the cathode resistor and by pass cap?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

Your schematic shows no CT on the secondary of the pwr xfmr, posted on December 29, 2020 at 15:58:07
Alpha Al
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I assume it's present, but not shown on the diagram, otherwise there would be no B+.

 

RE: Help needed, very confused., posted on December 29, 2020 at 17:59:54
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019



I think that I drew the filament schematic correctly. I used one Hammond 166M2 (save space) to supply the filaments on both the 27 and the 45. In retrospect that may be the problem.

On the 45, the bias resistor and by-pass capacitor are connected to the same pin.

 

RE: Help needed, very confused., posted on December 29, 2020 at 19:16:59
Tre'
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Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
So the 27's cathode resistor(and bypass cap) are in parallel with the 45's cathode resistor (and bypass cap).

That might be part of the problem. I think each tube needs it's own filament transformer.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

The 27 is indirectly heated, however there should be some current flowing, posted on December 29, 2020 at 21:26:45
Chip647
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Posts: 2633
Location: The South
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There is either a bad part or a bad solder connection. He should try drawing a complete schematic.

 

RE: Help needed, very confused., posted on December 29, 2020 at 22:09:35
Paul Joppa
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Location: Seattle, WA
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The 27 heater is not tolerant of much heater/cathode voltage difference; recommended practice is to tie the cathode to the filament voltage center-tap.

 

My mistake. For some reason I thought it was DH. nt, posted on December 30, 2020 at 07:05:07
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
.
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: My mistake. For some reason I thought it was DH. nt, posted on December 30, 2020 at 09:34:56
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019
I added a separate filament heater for the 45. Changed the bias resistor on the 6SF5 to a 2.2 k. Changed the bias resistor on the 45 to 1500 ohm.

The amp sounds much better. The voltage readings are the same. 190 plate, 4 mV at the top of the bias resistor and 0.7 at the grid.

I will go back through and check all of the solders and grounds again tonight.

I had a couple of spare 45s that appear to have gone bad over the years. For interest sake I put in a Soviet 2a3, and the amp came alive, loader, clearer and more sustain. I am wondering if my remaining 45 is bad.

 

RE: My mistake. For some reason I thought it was DH. nt, posted on December 30, 2020 at 09:49:26
Tre'
Industry Professional

Posts: 17263
Location: So. Cal.
Joined: February 9, 2002
"I am wondering if my remaining 45 is bad."

that could be.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: My mistake. For some reason I thought it was DH. nt, posted on January 1, 2021 at 08:24:12
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019
I finally got a break, on call for New Years and the newest COVID surge.

I took some readings with the 2a3 tube in place and remain confused. The amp sounds pretty amazing but the voltages don't make sense.

B+ is down from 250V to 195V. On the 2a3, plate is 106, cathode is 4 mV? From plate to cathode is 105 V.

 

The lower total voltage maybe because, posted on January 1, 2021 at 14:18:32
jedrider
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Location: No. California
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the load will pull it down, but unloaded it measures at the higher voltage. More capacitance may bring that level higher.

 

RE: The lower total voltage maybe because, posted on January 1, 2021 at 14:32:51
DOc95gab@gmail.com
Audiophile

Posts: 85
Location: Louisiana
Joined: November 1, 2019
I am guessing that you mean more capacitance in the power supply? Something like Gordon Rankin did in the Bugle's power supply.

 

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