Interesting finding concerning voltages non-HRM

[ElectronAvalanche]

Hi there,

been tweaking my 100W non-HRM a bit lately.

Straightforward non-HRM:

220k/150k and 3k3,2k2 plate/cathode resistors
4u7 cathode caps
220k with 500pf grid on V1b
Skyliner TS, 120pf and 270pf bright with SPTT switch
OD:
pre OD: .005uF

220k series, 100k trimmer
68k grid
0.01 coupling into 150k to Level (100k pot measuring 120k pot)
150k grid
0.005 coupling, 120k to Ratio (100k)
15pf on Master

Voltages:

V1a: 196V
V1B: 198V
V2a: 203V
V2b: 205V


This amp has Twin/Showman iron. The PT was salvaged from a Fender PA100W. These amps used Twin/Showman trannies.

I measured the heater voltage last night and low and behold I had 7V AC on the heaters. I went to a higher primary setting on the PT (from 230V to the 250 tap) and heater voltage dropped to around 6.6V.

The amp started to really breath with the lower voltages.

Now I have 182 - 185V on V1 and 190V on V2. The PI has lower voltages as well.

The sound got brighter, but rather pleasing. I think I will drop the MV bright cap now. With the Level pot of the OD really set low, one can get almost SSS like sounds. That woody violin-like Clean sound with some hair on it.

Overall I think that playing with global voltages (the power tube voltage dropped as well by 10Vs to 440V) is one of the most effective tweaks.

Be aware, that with a different set of transformers one might find other voltages for that sweet spot.

Cheers,

Dominik

[jelle]

Dominik,

I have also experienced that the sound gets sort of constipated when the heater voltages are a bit high. Restoring these voltages alone really helps the amps tone. Also the lifespan of the tubes is increased a lot when the heater voltage is 6.3V +/-10%. I do not recall where I have seen the graph showing lifespan and heatervoltage but it was convincing!
I'm glad you gave a good sound out of your amp.

BTW, how did you set up your OD entrance? I'm confused.

Jelle

[llemtt]

Dominik

you are right, heater voltage has some influence on preamp tube sound: the higher the voltage the bigger the clouds of electrons around the cathode, this is usually referred as a "bigger cathode" and changes the way the tube behave.

I never saw plate curves drawn at different heater voltages, altough it would be very useful, but I recently read an article showing different levels of distortion in an hifi preamp...

One of the most evident effects of a "bigger cathode" is that grid.cathode.capacitance is increased due to the reduced "actual" distance from cathode to grid; this turns out into poorer high freqs response and a darker sound.

Power tubes don't suffer the same effects because of their bigger size.

Lots of pro players use power conditioners or variacs or something similar to keep their voltage at their preferred values...

cheers
teo

[ElectronAvalanche]

Hi Jelle and Teo,

old Hammond organs used 5Vs for the tube heaters albeit the gospel says that too low a heater voltage will damage the tube as well. Yet I have tubes from a C2G Hammond from 1954 that sound amazing and are almost as good as new.

I do not know how much influence the lower heater voltage had on sound given that all voltages dropped when I switch the primary taps. Maybe I should experiment a bit with that.

@Jelle:

OD entrance:

from relais to 0.005 uF into 220k series resistor into 100k trimmer with one leg to ground, the other input, wiper to grid resistor.

Cheers,

Dominik

[jelle]

Thanks for the info.

FWIW, I have had a 12.6V DC heater system in an earlier ODS build that I did and initially the voltage was too high since it was more like 14 or 15V. The sound became constipated because of this. Since I did not have the parts to make it 12.6V, I just wired it as 6.3V AC and it sounded great and the hum was not an issue.

[heisthl]

I'm wondering if the better tone came more from the lower plate voltages than heater voltages. I've noticed this same effect just by changing dropping string values.