#40 resistor choice

[erwin_ve]

Hi Fred, sounding awesome! Liked your strat interpretation of that Scott Henderson tune.

Erwin

[fred.violleau]

@ian @Erwin, thanks guys, I really enjoy the build.

But speaking of voltages... I seem to be far away from the goal
Erwin's build had the following :

Voltages;
B1: 440v
B2: 437v
B3: 413v
B4: 283v
B5: 276v

V2b: 180v
v2a: 181v
v1b: 173v
v1a: 169v

bias: 30mA

Mines are as follow : (almost 40v shorts all the time)
Voltages;
B1: 399v
B2: 396,5v
B3: 379v
B4: 246v
B5: 238v

V2b: 157v
v2a: 160v
v1b: 145v
v1a: 150v

bias: 55,8mA so around 72% plate dissipation for 6L6GC, probably need to cool it a bit for the tubes longevity

Power transformers specs are :
Primary : 120 VAC @ 60 Hz (Black - White)
Secondary 1, red-red : 495 VDC unloaded, 440 VDC @ 65.7 mA DC, 388 VDC @ 200 mA DC
Secondary 2, brown-brown : 53.6 VAC @ 1.58 A AC
Secondary 3, green-green : 6.6 VAC @ 3.75 A AC

I thought it would have been OK, but I guess the PT is coming short and that is probably why it is getting hot as well...
Despite these, the amp sounds really good, a bit on the dark side and I will add the 1uf cap accent as alway on since it brightens the amp just a little.

Any thoughts?

Fred.

[dbharris]

Hi Fred, those clips sounded great! Good luck with getting the voltages dialed in.

-Dan

[bepone]

...
Despite these, the amp sounds really good, a bit on the dark side and I will add the 1uf cap accent as alway on since it brightens the amp just a little.

Any thoughts?

if you increase preanp voltages you can return the brightness, but also the vintage pihers can lead to dark sound..

[fred.violleau]

I think I will try and cheat the voltages with the variac and blast it to the max.
Funny enough my mains measured 117V when in summer it is around 124v.
I guess winter takes a toll on the overall power lines here in Canada.

I will try and shoot for the max current and try to hear the difference. May be able to gain 15 to 25 volts if I am lucky...
Brightness is OK to add, but I really enjoy the richness of the sound as it is now...
I have an old Bassman PT that I was going to use in another build, that I could probably try to hook up.

Question : Do I have to remove all the connections on the secondary side of the actual PT or could I plug my second PT paralleled to the one inside thew build, and only hook the HT to get voltages on par with what should be on the B+ line ? I need to ground the paralleled transformer, but is there any other recommendation ?

Thanks,

Fred.

[Bombacaototal]

Same here, colder days are around 235V and warm around 246V. I noticed that wet days measure different than dry ones too. The SSS style amps are so sensitive to the mains voltage (and bias) at least to my ears that I always try to keep my mains around 240V (+ - 2V)

[Richard1001]

@ian @Erwin, thanks guys, I really enjoy the build.

But speaking of voltages... I seem to be far away from the goal
Erwin's build had the following :

Voltages;
B1: 440v
B2: 437v
B3: 413v
B4: 283v
B5: 276v

V2b: 180v
v2a: 181v
v1b: 173v
v1a: 169v

bias: 30mA

Mines are as follow : (almost 40v shorts all the time)
Voltages;
B1: 399v
B2: 396,5v
B3: 379v
B4: 246v
B5: 238v

V2b: 157v
v2a: 160v
v1b: 145v
v1a: 150v

bias: 55,8mA so around 72% plate dissipation for 6L6GC, probably need to cool it a bit for the tubes longevity

Power transformers specs are :
Primary : 120 VAC @ 60 Hz (Black - White)
Secondary 1, red-red : 495 VDC unloaded, 440 VDC @ 65.7 mA DC, 388 VDC @ 200 mA DC
Secondary 2, brown-brown : 53.6 VAC @ 1.58 A AC
Secondary 3, green-green : 6.6 VAC @ 3.75 A AC

I thought it would have been OK, but I guess the PT is coming short and that is probably why it is getting hot as well...
Despite these, the amp sounds really good, a bit on the dark side and I will add the 1uf cap accent as alway on since it brightens the amp just a little.

Any thoughts?

Fred.

If it sounds good, it is good. And it sure sounds good!

According to the voltages, bias current and specs of your transformer, the B+ should go up to the same values if you set the bias current to 30mA / tube like you say Erwin did.

30mA would seem kind of cold at 440V (44%), but the biascurrent is just a number. What realy matters is the sound. Colder biased power tubes can actualy sound better when used with overdrive. Clearer and less mushy (is that a word?). Since you say the amp could use a little more clearity, it could be a good idea to lower the bias somewhat. Just dial it in where it sounds right to you.

In a class A/B amplifier you could look at the bias current setting as the range in which the power amp operates in class A and gradualy moves more towards class B operation when more power is needed from the amp.

Don't forget to measure your mains voltage with a true RMS meter. If it is lower than 120V your secondary voltages will be lower also.

[Charlie Wilson]

Fred, amp sounds great. I am curious about the 27k dropping resistor. Hmmm, never mind. That is the value on the layout.
CW

[fred.violleau]

Hey Richard1001,

Thanks for the advices. I checked and cranked my variac up to 130 VAC and voltages fall inline.
My mains was a low 117 yesterday ...

Now for the bias, I have 43 VAC current available on the PT.
I am using a 3k3 droping resistor and I measured 65mv on the 1ohms resistor to ground from pin 8
I have a 27k on the bias pot and it is maxed out.
when I turn in to the other side, the bias goes higher.

Which way would it be the most efficient ?
higher resistor on the pot to ground ? or smaller dropping resistor?

Fred.

[fred.violleau]

@Charlie Wilson, thanks a lot, I hope the build pays respect to your wonderfull generosity, I really enjoy it.
What would be your point on the 27k from the layout ? (just curious )

Fred

[Charlie Wilson]

Oh, yeah. I had #54 around for a bit which is a 3rd Gen. (sort of) and it had a 22k dropper. You could brighten up the preamp by lowering the 27k dropper but you may lose some of that cool organic feel of the lower voltages. I can't tell whether you have the 2nd Gen Hybrid dual master or not, but if you do, you could put a master bright cap on the clean side. You may want to consider a straight presence control rather than the accent if you are going to leave the presence cap in the circuit all of the time. You can play around with presence cap values and NFB resistors. You could try 1/2w Pihers or AB (10%) on your grids and maybe the OD. They are a bit brighter than the 1w. Finally, I noticed on #54, Dumble bypassed the 1 meg OD out resistor with a 25pf ceramic. Maybe something to try.
CW

[fred.violleau]

Oh, yeah. I had #54 around for a bit which is a 3rd Gen. (sort of) and it had a 22k dropper. You could brighten up the preamp by lowering the 27k dropper but you may lose some of that cool organic feel of the lower voltages.

I love those darker mid rich tones, good to know that is one way to brighten up the amp.

I can't tell whether you have the 2nd Gen Hybrid dual master or not, but if you do, you could put a master bright cap on the clean side. You may want to consider a straight presence control rather than the accent if you are going to leave the presence cap in the circuit all of the time. You can play around with presence cap values and NFB resistors. You could try 1/2w Pihers or AB (10%) on your grids and maybe the OD. They are a bit brighter than the 1w. Finally, I noticed on #54, Dumble bypassed the 1 meg OD out resistor with a 25pf ceramic. Maybe something to try.
CW

I have implemented a MV ala 102 and 183 : 1Meg volume pot out of the relay into the FX Loop, and I have an OD level and Master. I have added a bright cap on a MV on a previous build, subtle, yet interesting.

I read that presence cap is divided in 2 camps : 1uf and 2uf. On this build I have a 1uf tantalum, what 2uf would do ?

I will definetly try the 1Meg resistor bypass out the od as it is an easy clip in for an A/B test.

Will report during the day on those

Fred.

[Richard1001]

Hey Richard1001,

Thanks for the advices. I checked and cranked my variac up to 130 VAC and voltages fall inline.
My mains was a low 117 yesterday ...

Now for the bias, I have 43 VAC current available on the PT.
I am using a 3k3 droping resistor and I measured 65mv on the 1ohms resistor to ground from pin 8
I have a 27k on the bias pot and it is maxed out.
when I turn in to the other side, the bias goes higher.

Which way would it be the most efficient ?
higher resistor on the pot to ground ? or smaller dropping resistor?

Fred.

I'm not sure if i understand what you mean but 43 volts AC on the brown and brown wires is about 10 volts lower than the transformer specs you posted.

65mV over the 1 ohm cathode resistor is a high current for one tube and you should be able to lower it.
If the bias voltage can't be set more negative (maxed out like you said) you need to adjust the bias voltage supply.

What is the value of the cap you are using?
You could check the voltage drop over the 3K3 resistor by first measuring the maximum negative bias voltage with the resistor in place (on the cap) and after this short the resistor and measure the maximum negative voltage again (on the cap), and substracting both.

From there you can see what is the best way to go

[fred.violleau]

Sorry guys for the late reply, been busy yesterday and could not work on the amp.

@Richard1001 yes when I took the measurements, my mains was a low 117, and i had 43v on the bias tap instead of 50v.
My setup is as follow : 3k3 dropping resistor > 1n4007 diode > 47uf 160v (as the original Dumble layout)

Capture d’écran 2022-02-09 à 15.54.33.png

Now on the HRD schematic, I see they used 1.5k dropping resistor, and 22 uf reservoir cap and 100k resistor on the bias pot
They are also using this to power the relays.

Capture d’écran 2022-02-09 à 16.05.05.png

Here are the bias readings today :
after 3k3 : -36.7V
when 3k3 is bypassed : -53V

I put in a 1k5 instead of the of 3k3 and it brought me back to -43V at reservoir cap with mains at 119V

Here the new voltages : (still a little low)

Mains : 119V

B+1 : 418,8V
B+2 : 417,4V
B+3 : 398V
B+4 : 257,8V
B+5 : 249,9V

V1A : 168,8V
V1B : 166,3V
V2A : 168,8V
V2B 166,3V
V3 392 / 393,8V

Bias supply : -43V
and I get 37,2mv and 37,9mv on the 1 ohms resistor of the power tubes which seems to be in the right range now.

Thanks for your support !

Fred.

[Dr d]

I have implemented a MV ala 102 and 183 : 1Meg volume pot out of the relay into the FX Loop, and I have an OD level and Master. I have added a bright cap on a MV on a previous build, subtle, yet interesting.


For these amps, it is best to implement the Bluesmaster or Hybrid master volume setup otherwise you are going to load the OD incorrectly. See attached schematic.