Valve Jr Upgrade.

Tube or solid-state, this section goes to eleven!

Valve Jr Upgrade.

Postby steben » 24 Mar 2020, 18:23

OK.
I know.
I stayed with the single ended concept.

Got me a 125ESE just in time before lockdown. Wired some mods, JJ tubes and Bam! great improvement in sound.
90623729_633005710851666_7797463742354030592_n.jpg

Schematic as-built:
VJR3 mod SB.jpg


Now I wonder whether a blocking resistor before V2 after the volume pot might smooth the sound even more.
Is it worth it?
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Re: Valve Jr Upgrade.

Postby plush » 24 Mar 2020, 18:33

Is it me, or you are running you heaters little bit too hot from DC (8.3 vs 6.3)? Won't this be an issue for your tubes?

steben wrote:Now I wonder whether a blocking resistor before V2 after the volume pot might smooth the sound even more.
Is it worth it?


It depends on your smoothness criteria.
You can calculate it as lpf considering miller capacitance of your tube stage.
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Re: Valve Jr Upgrade.

Postby Ben N » 25 Mar 2020, 08:49

plush wrote:Is it me, or you are running you heaters little bit too hot from DC (8.3 vs 6.3)? Won't this be an issue for your tubes?
Does a DC filament supply really offer a noticeable improvement over a well constructed AC supply (twisted leads, virtual center tap, DC reference)?
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Re: Valve Jr Upgrade.

Postby plush » 25 Mar 2020, 09:53

Ben N wrote:Does a DC filament supply really offer a noticeable improvement over a well constructed AC supply (twisted leads, virtual center tap, DC reference)?


IMO it makes life way more easier (no need for hum balancing, cable-twisteroo and cathode elevation). So, for me it's an improvement.
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Re: Valve Jr Upgrade.

Postby steben » 25 Mar 2020, 12:09

plush wrote:Is it me, or you are running you heaters little bit too hot from DC (8.3 vs 6.3)? Won't this be an issue for your tubes?


It is the standard VJr circuit as is ...

(6.3 - 0.65) * 1.41 = +/- 8 V ... Unloaded.
I get around 7.2V

On the TDPRI.com forum I'v found:

Did some playing around earlier, measuring heater voltages on the amp. Stock, they're quite high, at 7.15v DC. Here's a quick list of voltages with different sets of tubes:
6.34v with EL34 and 1x 12AX7
6.19v with EL34 and 2x 12AX7
6.92v with 6L6 and 1x 12AX7
6.70v with 6L6 and 2x 12AX7
7.35v with 6V6 and 1x 12AX7
7.08v with 6V6 and 2x 12AX7
6.92 with 6L6WGB and 1x 12AX7
6.62 with 6L6WGB and 2x 12AX7
8.11v with 1x 12AX7
9.51v no tubes installed
7.15v with original EL84 and 12AX7


So ... yes. It should be somewhat lower to extend tube life. Guitar amps are simply designed over the limits everywhere, aren't they?
Current is huge (+/- 1 Amp), so any resistor would need higher power rating than standard in an instant.

Subtracting 0.5V would need 0.5V/1A = 0.5 ohms . 0.5 V * 1 A = 0.5W
-> 1 Watt resistor.
Is adding a diode in series an option? This would need a high current rating. 1N5408 has 3A.

plush wrote:It depends on your smoothness criteria.
You can calculate it as lpf considering miller capacitance of your tube stage.


Most calculations of examples lead to freqs well above 10k, yet the blocking distortion might be reduced - grid stopper function.
I could just do some trial and error of course.
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Re: Valve Jr Upgrade.

Postby plush » 25 Mar 2020, 22:44

steben wrote:
plush wrote:Is it me, or you are running you heaters little bit too hot from DC (8.3 vs 6.3)? Won't this be an issue for your tubes?


It is the standard VJr circuit as is ...

(6.3 - 0.65) * 1.41 = +/- 8 V ... Unloaded.
I get around 7.2V

On the TDPRI.com forum I'v found:

Did some playing around earlier, measuring heater voltages on the amp. Stock, they're quite high, at 7.15v DC. Here's a quick list of voltages with different sets of tubes:
6.34v with EL34 and 1x 12AX7
6.19v with EL34 and 2x 12AX7
6.92v with 6L6 and 1x 12AX7
6.70v with 6L6 and 2x 12AX7
7.35v with 6V6 and 1x 12AX7
7.08v with 6V6 and 2x 12AX7
6.92 with 6L6WGB and 1x 12AX7
6.62 with 6L6WGB and 2x 12AX7
8.11v with 1x 12AX7
9.51v no tubes installed
7.15v with original EL84 and 12AX7


So ... yes. It should be somewhat lower to extend tube life. Guitar amps are simply designed over the limits everywhere, aren't they?
Current is huge (+/- 1 Amp), so any resistor would need higher power rating than standard in an instant.

Subtracting 0.5V would need 0.5V/1A = 0.5 ohms . 0.5 V * 1 A = 0.5W
-> 1 Watt resistor.
Is adding a diode in series an option? This would need a high current rating. 1N5408 has 3A.


Please ignore my brain fart, i forgot to add diode drop to my calculations. 7.2 is pretty reasonable value.
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