Recent Posts

1

Tube Amp Building - Tweaks - Repairs / Re: 5E3 Bias

« Last post by stratomaster on October 24, 2025, 12:27:57 pm »

That's not a schematic.  Also, by replacing the 4.7k dropping resistor with a choke you've stripped the circuit of any screen protection.  If you're going to keep the choke, please add 1k screen resistors to each socket.  This will also buy a bit of compression to offset the increased voltage.

25V at pin 8 with a 250Ω resistor and approx 350V plate to cathode comes out to about 125% idle dissipation.  On a 1957 5E9-A Tremolux I recently restored, I had a similar situation.  Increasing the cathode resistor to 300Ω and using a 5Y3 brought the idle dissipation down to about 105% and the amp was very happy with that.

Also: move your standby switch to after the reservoir node.  This layout illustrates a hot switching arrangement that will lead to premature rectifier tube failure.

2

Tube Amp Building - Tweaks - Repairs / Re: 5E3 Bias

« Last post by shooter on October 24, 2025, 12:25:11 pm »

you could just make the 250 cathode R a 470, 500, 530, whatever you have close to double the 250. then re-measure VDC at pin 3 n pin 8


do the ball park math  ((plate VDC-cathode VDC) / Cathode R) / 2 (tubes) will yield total tube current including a few mA for G2
anything in the 35-45mA range is good for starts

3

Tube Amp Building - Tweaks - Repairs / Re: 5E3 Bias

« Last post by dpm309 on October 24, 2025, 12:01:39 pm »

I have attached the schematic for the Dumble 5E3 mods. The only difference is that I am using a choke instead of the 4.7K 2Watt resistor.  I am getting 374 VDC on pin 3, 379VDC on pin 4 and 25VDC on pin 8. B+:  382VDC, B+1 : 380VDC and B+2: 300VDC. Haven't tried swapping rectifier back to the 5Y3. I'll try swapping the rectifier after I can locate one.

4

Tube Amp Building - Tweaks - Repairs / Re: 6G15 Chassis Hole Layout

« Last post by roboDNA on October 24, 2025, 11:20:16 am »

Here's a thread that discusses this question on Surfguitar101:
https://surfguitar101.com/forums/topic/12532/?page=1#p171048

The spacing given is center-to-center, measured from the mounting hole on the left, so you would want to adjust accordingly.

This is what I was looking for, thank you!

Who should I order an official face plate from?  I could buy one.

5

Tube Amp Building - Tweaks - Repairs / Re: Seeking 18W enlightenment

« Last post by SEL49 on October 24, 2025, 11:08:42 am »

That is hard (for me) to understand.   While your plate voltage is lower, your cathode voltage is higher, which points to the tube, I guess.

That's normal. The plate voltage is lower because of higher current through the tube causing more voltage to be dropped across the plate resistor and that leaves less voltage to be dropped across the tube. Higher current means the cathode voltage will be higher.

Those resistor values are normally used on a 12AX7, not a 12AU7.

6

Tube Amp Building - Tweaks - Repairs / Re: Seeking 18W enlightenment

« Last post by acheld on October 24, 2025, 10:32:21 am »

That is hard (for me) to understand.   While your plate voltage is lower, your cathode voltage is higher, which points to the tube, I guess.   It is always possible the initially recorded voltages were wrong, but your cathode voltages are otherwise in line, so maybe not likely.  I guess you could check the 5K dropping resistor between C and D, but that would not account for what you're seeing.

I'm stumped; hopefully not chumped.

7

Tube Amp Building - Tweaks - Repairs / Re: Fender Blues Deluxe RI Sound bad

« Last post by J. Ignacio on October 24, 2025, 10:31:13 am »

Cap 47uf x 500v dead. I change it and now amp works fine.

8

Tube Amp Building - Tweaks - Repairs / Carvin Legacy 3 bias issue or design?

« Last post by EddyMunster on October 24, 2025, 10:08:09 am »

I am new to this forum so hello to everyone.

I have poked around the web for an answer to this question with no luck.

I have two Carvin Legacy 3 amplifier heads and I am getting the same results from both.

I set the bias to 100ma across the bypass switch per Carvin's method on both amps.

I did a comparison and noticed the power tubes are only running about 8ma per tube. I measured the 8ma using a bias probe at each tube socket.

I'm thinking they should be reading closer to 20ma per tube.

Is anyone familiar with these particular amps or has anyone seen this with Legacy amps?

I attached a schematic but I'm not very savvy about amps.

-Eddy

9

Tube Amp Building - Tweaks - Repairs / Seeking 18W enlightenment

« Last post by Lectroid on October 24, 2025, 10:08:00 am »

About a year ago I finished an 18W amp from by as circuit by tubenit. After initially getting it running, I documented the voltages at that time on the schematic below.  Those numbers on the schematic are a year old.

I re-measured the voltages yesterday:
A=334
B=323
C=288
D=271

   Plate      Cathode   Rp      Rk
------------------------------------------------------------------------
V1b:   90      2.87       100K       1.5K
V1a:   82      2.85       100K       1.5K

V2b:   312      12.06    Reverb   1.5K
V2a:   82      2.85       100K      1.5K

V3b:   194      1.77       82K      (1.5K eff/shared 820R)
V3a:   82      2.85       100K      (1.5K eff/shared 820R)

V4:   328      11.53   -      213R
V5:   328      11.53   -      213R


Here's my question.  When I compare the original voltage numbers from a year ago (schematic) with the numbers I got yesterday, they are close to what I recorded a year ago.  Overall, I'm happy with it.

EXCEPT--on the first stage tube V1.  V1b's plate voltage is 90 volts, or 38 volts lower than it was a year ago.  Likewise V1a's plate voltage is 82, or 40 volts below what it was a year ago.  See the schematic above for the original numbers. 

Why would the voltages for the V1 first stage now be so far below the voltages I recorded when the amp was just built?  All the other tubes' voltages are back close to those original values.   I've rolled a few tubes through there but voltages don't change with that.  I'm probably missing something obvious.  Can anyone point it anything else can I test?

10

Tube Amp Building - Tweaks - Repairs / Re: 18W odd power problems

« Last post by Lectroid on October 24, 2025, 09:32:08 am »

Thanks for all the help and suggestions. 

After a few days off doing life, I rebuilt the C and D filter caps' section in a better location. I must have re-worked some unknown bad connection because it fired right up.  It now is back to amping sound the way it used to except for one question I'll address in a new post.

Appreciate all the ideas and support!