Mojotone Vibro Champ - Biasing Help Please

[jewishjay]

I just finished my first build. It won't be my last, I had so much fun! It's a Mojotone kit, the Blackface Vibro Champ, and it works and plays, although it isn't very loud.

Let me describe the measurements I've taken and conclusion I've come to, and I hope you'll correct me if I'm wrong. First I measured the resistance of the cathode bias resistor for the JJ 6V6S and found 492 ohms. Then I switched the amp on, and found the voltage drop across it -25.5 Vdc. Finally, I measured the plate voltage to ground 382 Vdc. So I calculate the plate current to be 51ma and therefore the plate dissipation to be 19.5 watts. Too hot?

So I think I need to replace the 500ohm resistor with a 1k. Am I right? Am I wrong? Did I do something wrong? The amp plays fine, although I am unimpressed with its volume, but maybe thats a 7 watt amp for you. I don't know. What do you think?

[sluckey]

Let me describe the measurements I've taken and conclusion I've come to, and I hope you'll correct me if I'm wrong. First I measured the resistance of the cathode bias resistor for the JJ 6V6S and found 492 ohms. Then I switched the amp on, and found the voltage drop across it -25.5 Vdc. Finally, I measured the plate voltage to ground 382 Vdc. So I calculate the plate current to be 51ma and therefore the plate dissipation to be 19.5 watts. Too hot?

You forgot to subtract the 25.5v from the 382v so the actual plate voltage is only 356.5 making plate dissipation only 18.48 watts. But that's still too hot!

I'd replace the 500ohm resistor with a 1k and recheck the dissipation.

This is a common issue that we keep seeing over and over these days.

[shooter]

I don't know the kit schematic, each PA tube gets a cathode R or 1 cathode R for 2 tubes?

[jewishjay]

I don't know the kit schematic, each PA tube gets a cathode R or 1 cathode R for 2 tubes?

It's single ended.

[shooter]

thanks;read Champ, brain registered Verb 

[jewishjay]

I just got a reply from Andy at Mojotone. He says to play it for 10 to 15 hours and measure again before changing anything. So...ok....here goes....

[shooter]

always loved the melt down n testing 

[AmberB]

The B+ voltage does seem a bit high for a cathode biased amp.  It probably should be 350 or less.  If it had negative bias, it could run at a higher voltage because you could adjust the bias to keep within the specs of the tube.  With cathode bias, you're stuck with changing the bias resistor.
Does the amp have a tube rectifier?  I'm not familiar with the Mojotone amps...

[tubeswell]

The B+ voltage does seem a bit high for a cathode biased amp.  It probably should be 350 or less.  If it had negative bias, it could run at a higher voltage because you could adjust the bias to keep within the specs of the tube.  With cathode bias, you're stuck with changing the bias resistor.
Does the amp have a tube rectifier?  I'm not familiar with the Mojotone amps...

Mojotone Blackface Vibro-Champ Schematic

The HT winding on the 760 PT is advertised as 330-0-330VAC @75mA, so it should be a B+ of about 380VDC with a 5Y3 and a 20uF reservoir cap:

https://www.mojotone.com/amp-parts/Power-Transformers/Blackface-Princeton-Champ-Power-Transformer-EX


If its a Sovtek 5Y3G, the B+ could be a bit higher



The Fender Vibrochamp Schematic HT winding is 315-0-315VAC

[shooter]

the nice thing about SE, it's already has "hot" as it gets with no signal, give it signal and it cools off, so play hard 

[pdf64]

Rather than modify the response of a classic by shifting its bias away from centre towards cold, to my mind it makes better sense to restore the operating point to the original design, ie lower the HT.
That may be achieved by inserting a resistor between each plate of the rectifier and the PT's HT winding leg that feeds it.
About 270ohm 2 to 3W MO may be a good starting point to do that.

[tubeswell]

Rather than modify the response of a classic by shifting its bias away from centre towards cold, to my mind it makes better sense to restore the operating point to the original design, ie lower the HT.
That may be achieved by inserting a resistor between each plate of the rectifier and the PT's HT winding leg that feeds it.
About 270ohm 2 to 3W MO may be a good starting point to do that.

Or a 15-0-15 Vac bucking PT


Edit: See Attachment

[jewishjay]

Rather than modify the response of a classic by shifting its bias away from centre towards cold, to my mind it makes better sense to restore the operating point to the original design, ie lower the HT.
That may be achieved by inserting a resistor between each plate of the rectifier and the PT's HT winding leg that feeds it.
About 270ohm 2 to 3W MO may be a good starting point to do that.

alright, so i have this 1k 5w ceramic resistor that i was going to swap into the bias position, but now i'm contemplating  using it to drop the B+ instead. @ 50ma seems like that should take me from 382 to around 330.

thoughts?

[sluckey]

Do it both ways. Tell us what you think.

[labb]

I would change the cathode resistor to the 1K, recheck the plate voltage, recalculate,hope to get dissipation down to 14 watts. If so let her bump. play the heck out of it....be sure to recheck the plate voltage, it is going to increase.
Or just play it as is. Go to Schematic Haven, look at the schematic for the Vibrochamp. it has 420 volt to the OT which would put about 415 on the plate. The cathode resistor is 470 ohm and they show a voltage drop of 24 volts...all is right about where you are running. Fender ran some of the SE amps hotter than hell. This is one of them.

[Ed_Chambley]

To answer your question about the Volume of a 7 watt amp.  Get anyone who doesn't play guitar.  Turn the amp up full volume.  Ask them to stand in front and you play a hammer on and pull off at fret 17 to 20 in the little e string and ask them if it is loud enough.  If they look at you like you lost you mind, amp volume is just fine!


If they don't flinch, something is definitely wrong.

[jewishjay]

Get anyone who doesn't play guitar.  Turn the amp up full volume.  Ask them to stand in front and you play a hammer on and pull off at fret 17 to 20 in the little e string and ask them if it is loud enough.

lol. thanks for that. I'm used to my Fender Blues Deluxe and Carvin X100B. No frame of reference for these little guys.

[pdf64]

The SPL that an amp can kick out is totally dependent on the sensitivity of the speaker / cab it’s plugged into.
I guess that your other amps get a lot quieter if plugged into the 8” Mojo speaker?

[tubeswell]

Or a little variable B+ reducer in 4 parts


(Also I added a schematic for the bucking PT suggested in my earlier post)

[sluckey]

Hmm. Where's the bucking PT?

[tubeswell]

Hmm. Where's the bucking PT?

Post #11 attachment

[Ed_Chambley]

Just make one of these and you will have something you can use on any amp you have.  Check your filament voltage as you reduce.

[Ed_Chambley]

Also use a variac (Autoformer) and see if you can get the operating conditions within reason.  What?  You don't have one?

[jewishjay]

thanks for the help everyone, here's a quick demo of the completed amp.

[shooter]

so what was the final dissipation you settled on?

[jewishjay]

so what was the final dissipation you settled on?

I havent changed anything yet, but at last measurement its running around 17 watts. IF I were to reduce it to 14 Watts, how might that affect the tone/breakup/volume? Does higher plate dissipation make the amp breakup earlier or later?

[pdf64]

so what was the final dissipation you settled on?

I havent changed anything yet, but at last measurement its running around 17 watts. IF I were to reduce it to 14 Watts, how might that affect the tone/breakup/volume? Does higher plate dissipation make the amp breakup earlier or later?

Per se, tube dissipation affects nothing other than tube life.
As HT is increased, for a given load, power output will increase; plate current will also increase, thereby increasing stage gain.
As 'breakup' seems somewhat nebulous, I dunno how it might be affected by the above?

[jewishjay]

Per se, tube dissipation affects nothing other than tube life.
As HT is increased, for a given load, power output will increase; plate current will also increase, thereby increasing stage gain.
As 'breakup' seems somewhat nebulous, I dunno how it might be affected by the above?

Most people understand "breakup" to be the opposite of "headroom." But I agree that musicians use lots of vague meaningless terms to describe sound. Warm, round, full, fat, crunchy, smooth....None of these words really make sense in terms of audio, but we know what they mean.

It seems to be the general consensus on this forum that plate dissipation has almost no meaningful impact on breakup vs clean headroom. So everything I've heard and read about that is just sales pitch and snake oil?

[shooter]

IF
headroom = clean
AND
breakup = distortion
THEN
they are valid "design" considerations
NOT
snake oil
 

[pdf64]

IF
headroom = clean
AND
breakup = distortion
THEN
they are valid "design" considerations
...

Fine, but then what does 'more headroom' mean? Or 'earlier breakup'?
Techs think of these things from the perspective of the power amp's performance, eg so more clean power output = more headroom.
Whereas some guitarists view things from the perspective of the amp's control panel, eg so swap input tube from 12AX7 to 12AY7 (thereby reducing gain and so allowing a greater clean range of control from the volume knob) = more headroom.
And 'earlier breakup' may mean with regard to the volume control setting, rather than the amp's power output.
And as both gain and power output are increased with increasing HT voltage, the breakup point on the volume knob may remain broadly the same.
Hence it's preferable to use clearly defined terminology that refer the measurable characteristics, as those 'ifs' and 'thens' may not be 'givens' 

[shooter]

We're on the same page, I just read right to left 
IF the guitarist wants more "headroom" it's a tech/builders job to translate that to E-tronic speak.
once that's complete, the builder/designer/tech comes up with the "how to get there".
the guitarist is an artist, so you can't really make them happy until you translate back to their language, then they'll still complain, but come back for more.
hope that helps 

[jewishjay]

IF
headroom = clean
AND
breakup = distortion
THEN
they are valid "design" considerations
NOT
snake oil
 

YES, so how would you answer the question "to get early breakup (more dirt) do you raise or lower the plate dissipation?"

[shooter]

It's not the dissipation you change (talking SE)
it's the point at which clipping occurs
think of the voltage at the cathode of a PA tube as 1 limit
think of the plate volts on same tube as the other limit


using 200 plate, 20 cathode example
the signal hitting the tubes grid is <20 it stays clean.... as long as the PA tube gain doesn't push the amplified signal to >200  (It's more complicated than that but close enough for now)


so for more "dirt" the fast answer is to give the PA tube a bigger signal that causes distortion
the more complex way is to adjust Vk Vp to "distort" with a "given" grid signal

[pdf64]

...so how would you answer the question "to get early breakup (more dirt)...?"

Do you want more distortion at the same, or lower (or higher) audio power output?

[PRR]

There's little reason to run a 6V6 at 17 Watts Pdiss. (Exception: Fender's recent Champs do that to be SURE they are the loudest SE amp in the showroom.)

Get near 99% of 14W and V/I about equal to nominal load. This may require dropping B+ rather than jacking the cathode.

"Headroom" arguments are usually fruitless. Is it relative to pick? relative to drummer?

[HotBluePlates]

... its running around 17 watts. IF I were to reduce it to 14 Watts, how might that affect the tone/breakup/volume? ...

... tube dissipation affects nothing other than tube life. ...

Most people understand "breakup" to be the opposite of "headroom." But I agree that musicians use lots of vague meaningless terms ...

It seems to be the general consensus on this forum that plate dissipation has almost no meaningful impact on breakup vs clean headroom. So everything I've heard and read about that is just sales pitch and snake oil?

Here's the Concrete, JewishJay:

    -  Right now small small-ish bias voltage results in a plate current that, along with plate voltage, gives high dissipation

    -  Turning down plate voltage is harder to do, so we'd turn down plate current (with a larger bias voltage)

    -  The output tube distorts for-sure when the incoming signal is as-big as the bias voltage.  Therefore:

    -  Biasing the output tube cooler will allow the output tube to accept a larger drive signal before it surely-distorts.

The subjective result is the the player notices the Volume knob has to be turned higher before the amp distorts.  For many reasons, the amp probably doesn't deliver a bigger-power to the speaker when it distorts.  Player says "More Headroom" but Engineer says "That's all Perspective; Not Much Changed."

... so how would you answer the question "to get early breakup (more dirt) do you ...?"

When I was in the Army they said, "You gotta be 10% smarter than the equipment you're working with."  When dealing with guitarists, "You gotta figure out the question they should have asked rather than the question they're actually asking."

You want more breakup/distortion from your Vibro Champ?  I've gotta know that the output tube distorts first in that amp.  I've also gotta figure out that you probably want distortion with less sound coming from the speaker (cause you can already use umpteen-many boost pedals to slam the output tube harder).

So I have to decide the player really wants to know, "How do I get less output power from this amp, so it distorts while being quieter?"  Oh... and we have to decide that "Use an Attenuator" was not a desired response.

So what to do?  The shortest path to less-power from the output tube is Lower the Screen Voltage.  That reduces plate current, reduces the maximum/peak plate current the tube can manage, and so knocks down power output.  "Smaller screen voltage" also means "smaller bias voltage" for the tube, so a smaller drive signal will be enough to make the tube distort.

Will it Sound Good?  Maybe... that's what prototyping is for.  What's the "Right Way" to reduce the screen voltage?  Probably depends on how much compression is acceptable to the player (another question they didn't think to ask that you have to ask & answer).


Like a lot of things in life, the Simple Answer is either Wrong, Misleading, or has a Lot of Unstated Background built in...

[shooter]

+1

 


i'm going with unstated 

[HotBluePlates]

... plate dissipation ... impact on breakup vs clean headroom. So everything I've heard and read about that is just sales pitch and snake oil?

On a different note, what "sales pitch" focuses on plate dissipation?  Most marketing departments have no idea what that is (and know the guys reading the ad copy don't know, either).

Within one amp (or design) plate dissipation doesn't have a lot of bearing on amp power, distortion, etc.  Earlier I alluded to how it will change.

Between amp design plate dissipation can have a profound impact on amp power, distortion, etc.

     -  Think of the difference between a tweed Deluxe and a MusicMan RD-65.  One idles near-max, while the other idles near-zero dissipation.

     -  The one idling near-max doesn't have a large increase of plate current during signal.  It also doesn't need much drive signal (to run into distortion), but then again it can't output A LOT of power.

     -  The one idling near-zero has an enormous increase of plate current during signal; this is the reason it must idle cold: to keep the tubes from melting when driven.  Drive signal required is much larger, though proper preamp (or perhaps driver-stage) design might hide this fact.  This amp can deliver A LOT of power output.

But!!  Plate dissipation is far from the only difference between these two amp designs.  Supply voltage, power supply design, output transformer loading, bias method, etc are all gonna be very-different.



The interesting bit is the less current-change there is in use, the hotter you can run the amp at idle, and this is correlated with lower power output, and easier drive requirements.