GAStan wrote: ↑Thu Oct 05, 2023 9:02 pm
I've noticed several values used for the artificial ground on the heaters. 100 ohm common on other amps, 120 ohm for #124 and 150 ohm on #183. Is there any guidelines for this resistor value? I ask because I do have several 470 ohm 1 watt that I could put in.
Quote from Merlin's 'Designing Power Supplies...':
"They should be small-valued to encourage the shunting of transformer leakage current to earth and to avoid adding unnecessarily to Rhk, so values of 100 [Ohm] 1/2W to 220 [Ohm] 1/4W are usual."
And, from 'Designing Valve Preamps...':
"Values of 100 [Ohm] to 1 k[Ohm] 1/4W are usual. This has the advantage that if an anode-to-heater short should occur between the pins of a valve... the resistors are likely to fuse before any serious damage is done."
In Merlin's view at least, smaller power dissipation is better. Under normal operation, the resistors only have to dissipate a small amount of power - run it by Ohm's law and the 100R only need to handle around 0.4W and 470R around 85mW - using lower rated resistors than the 1,2 and 3W we have been discussing allows the resistors to perform double duty as fuses.
Check the datasheet for your tubes' allowable bias resistance: Rhk (max). The Vhk (max) should be within limits with a 470R. I don't think it would do any harm in the short term but you might be better waiting until the 120R arrive from Mouser.
GAStan wrote: ↑Thu Oct 05, 2023 9:02 pm
I've noticed several values used for the artificial ground on the heaters. 100 ohm common on other amps, 120 ohm for #124 and 150 ohm on #183. Is there any guidelines for this resistor value? I ask because I do have several 470 ohm 1 watt that I could put in.
Quote from Merlin's 'Designing Power Supplies...':
"They should be small-valued to encourage the shunting of transformer leakage current to earth and to avoid adding unnecessarily to Rhk, so values of 100 [Ohm] 1/2W to 220 [Ohm] 1/4W are usual."
And, from 'Designing Valve Preamps...':
"Values of 100 [Ohm] to 1 k[Ohm] 1/4W are usual. This has the advantage that if an anode-to-heater short should occur between the pins of a valve... the resistors are likely to fuse before any serious damage is done."
In Merlin's view at least, smaller power dissipation is better. Under normal operation, the resistors only have to dissipate a small amount of power - run it by Ohm's law and the 100R only need to handle around 0.4W and 470R around 85mW - using lower rated resistors than the 1,2 and 3W we have been discussing allows the resistors to perform double duty as fuses.
Check the datasheet for your tubes' allowable bias resistance: Rhk (max). The Vhk (max) should be within limits with a 470R. I don't think it would do any harm in the short term but you might be better waiting until the 120R arrive from Mouser.
All of this makes sense to me. Also I did unwittingly prove that a 120 ohm 1 watt resistor will, at least in my case, act to discharge the high voltage/current to ground keeping it off of the heater windings. I believe it acted more like a varistor than a fuse.
Received the 120R resistors and put them in. Installed them on tube side of voltage reducing resistors.
Wall voltage today is 120vac. Heater voltage is 6.9 from xfmr and 6.3 on tubes. All is good.
Biased tubes to 430v at B+1 and 38mA, 16.3 Watts around 55% of a 30 watt 6L6 (if I'm calculating right). This is using B+1 not actual plate voltage. I want to keep it biased cool until my wall voltage is back up to its regular 125-128 and check it then. Or I might use the variac...
Will plug in a guitar and try it out in a little while.
With nothing plugged into the input it's silent with volumes at 0, huge improvement with the new tubes.
The noise I refer to in the following is a combination of hiss, or white noise, and hum. I believe the hum is 60 cycle but will verify with an o'scope in the near future.
Following has Master volume at 50% or higher. Noise gets loud, too loud to carry on a conversation without shouting, at max volume settings. Still nothing plugged into input.
With OD off noise and volume maxed, PAB off, TMB all at 0 noise is very low level, barely a whisper. Hiss is variable with Treble, hum variable with Bass, Middle has no effect. In fact Middle has zero effect period on anything, any setting.
With CL volume at zero and OD on, OD volume maxed but Drive at 0 its quiet. Noise, hum and hiss, variable with Drive getting very loud. With Drive up enough to hear noise, noise level can be increased with CL Volume. With CL volume up T and B vary hiss and hum respectively. OD trimmer, 250k, is at 50%.
I did plug in a guitar and play for a while. At low volumes noise is not bad. It sounds good but I already have noticed some things I'd like to work on. I will save those for other posts after I address the noise, targeting primarily the hum.
GAStan wrote: ↑Sat Oct 07, 2023 12:46 am
Swapped in another tube for V1 and hum is MUCH better. I can vary the hum by touching the chassis or just waving my hand over it.
But this tube is microphonic. I'll continue to roll tubes until I find the best combination of the tubes I have available.
What is an acceptable loudness level of noise at full volume? Whisper? Normal conversation? Shouting level?
I never test mine at full volume like that because I will never play the amp that way. I usually set it for the loudest I will play the amp and then 1 notch more. If the background noise is between whisper and normal conversation in OD mode, I would call it acceptable. My clean channel is very quiet, OD has a little hiss.
You may gain some mileage by adjusting the OD entrance now. I think 50% may be a little higher than optimal...could contribute to buzziness and hiss in OD.
GAStan wrote: ↑Sat Oct 07, 2023 12:46 am
Swapped in another tube for V1 and hum is MUCH better. I can vary the hum by touching the chassis or just waving my hand over it.
But this tube is microphonic. I'll continue to roll tubes until I find the best combination of the tubes I have available.
What is an acceptable loudness level of noise at full volume? Whisper? Normal conversation? Shouting level?
ideally with nothing plugged, low to very low noise, on clean must be almost nothing
Are you running a ground from the input jacks or relying on the chassis contact as a ground? You may want to try isolate both NOR and FET inputs and run a dedicated ground wire.
I also don't see one side of the input jack coax grounded. On #0124 HAD grounded it at the jack but in most other amps he grounded it at the V1 tag strip.
ijedouglas wrote: ↑Sat Oct 07, 2023 2:57 pm
Are you running a ground from the input jacks or relying on the chassis contact as a ground? You may want to try isolate both NOR and FET inputs and run a dedicated ground wire. .
Chassis ground. I'll look into some isolated input jacks.
ijedouglas wrote: ↑Sat Oct 07, 2023 2:57 pm
I also don't see one side of the input jack coax grounded. On #0124 HAD grounded it at the jack but in most other amps he grounded it at the V1 tag strip.
Coax is grounded at input jack and isolated at V1. The coax is 20awg solid center conductor but spiral wound shield. It wasn't until I was well past installing these that I read about using foil shielded coax. If I change the input jacks to isolated I will change the coax at the same time and ground it at V1
I've been rolling tubes but it is taking a while. Some of the tubes are brand new and need some time to burn in. The tube I spoke about previously (Tung-Sol ECC803S gold pin) had bad hiss when initially installed but was very quiet after a few hours run time. But it was a little microphonic, every time I moved a switch it sounded like I had tapped a wine glass through the speaker. Lightly tapping the tube with the tip of my fingernail created the same effect but louder.
Currently it has a TAD 7025 High Grade in it. Absolutely no hum but a fair amount of hiss/static with an occasional pop. However it only has about 1/2 hour run time.
GAStan wrote: ↑Sat Oct 07, 2023 12:46 am
Swapped in another tube for V1 and hum is MUCH better. I can vary the hum by touching the chassis or just waving my hand over it.
But this tube is microphonic. I'll continue to roll tubes until I find the best combination of the tubes I have available.
What is an acceptable loudness level of noise at full volume? Whisper? Normal conversation? Shouting level?
I never test mine at full volume like that because I will never play the amp that way. I usually set it for the loudest I will play the amp and then 1 notch more. If the background noise is between whisper and normal conversation in OD mode, I would call it acceptable. My clean channel is very quiet, OD has a little hiss.
You may gain some mileage by adjusting the OD entrance now. I think 50% may be a little higher than optimal...could contribute to buzziness and hiss in OD.
Good luck!
-Dan
Thank Dan, for letting me know how you check for noise and what you consider acceptable. Exactly what I'm looking for.
The 50% was just an arbitrary setting I put the trimmer at prior to initial power up. I'll set it soon.
