Power supply Dropping resisters question, Silverface Fender Bandmaster

Ossyrocks

Hi chaps, I’m going to ask here about this one.

Many years ago, I slotted a Bandmaster Reverb chassis into a 2x10 combo cab and blackface modded it. At the time I wanted something spanky and loud to gig fairly big rooms with, so it got 7581A’s and Eminence Rabin Cajuns.

I sold the amp about 8 years ago, but this week I got it back. I’ve looked back at my work, and I’m quite pleased with what I did with it originally. However, times have changed and I’ve got a bit more sensible, so I’m going to try to make it a but softer and quieter to perhaps get the sweet spot down to where I could use it in smaller rooms.

I’m swapping out the Eminences for something more vintage with less sensitivity and I’ve fitted some small bottle Philips 6L6WGBs. 

The one thing I didn’t change years ago were the values of the dropping resisters under the cap can. I kept the 4.7k + 1k combination for max preamp voltage and headroom. I’m thinking I could change these for the 10k + 10k combination shown on the blackface Vibrolux schematic, but I’m hesitating as I’m lacking a little confidence to do it, as I don’t know if I can or should with this power transformer.

Basically, can I do this, and would it achieve what I think it might?

cheers,
Rob

gjonesy

I’m not technically minded enough to recommend the resistor changes but I’d do one swap at a time and see how you feel. You’ll also get a feel for which gives the biggest change. 

I’m running an attenuator, so unless the amp is also going in for service this might be something to consider as you could use it on other amps too.

ICBM

That's perfectly safe, since the result will be to lower the preamp power supply voltages slightly and won't load anything more heavily at all, but it will make next to no (or actually no) difference to the headroom which is largely limited by the power amp.

If you need noticeably less power you need an attenuator or a post-PI master volume.

Ossyrocks

ICBM said:

That's perfectly safe, since the result will be to lower the preamp power supply voltages slightly and won't load anything more heavily at all, but it will make next to no (or actually no) difference to the headroom which is largely limited by the power amp.

If you need noticeably less power you need an attenuator or a post-PI master volume.

Thanks for the reassurance John.

Even if the difference is only slight, I’m hoping the combination of all the changes will end up where I want it to be.

I’ll get the iron warmed up now! 

PeteC

You probably already know but if you’re increasing the value of the dropping string it will dissipate more power and so the ps resistors need to be rated for the wattage    I typically use at least 2 watts but higher is also good ! 

Ossyrocks

PeteC said:

You probably already know but if you’re increasing the value of the dropping string it will dissipate more power and so the ps resistors need to be rated for the wattage    I typically use at least 2 watts but higher is also good ! 

Yes, I’m going for 3 watt metal oxides.

Keefy

You've probably already thought of this, but how old are the filter caps? Might be time to renew them.

Ossyrocks

Keefy said:

You've probably already thought of this, but how old are the filter caps? Might be time to renew them.

I replaced those when I originally built the amp about 10 years ago. I usually use Sprague Atoms, but for some reason at the time I used F&T’s, but I think they’ll still be ok for a while.

With lockdowns and having more time in my hands I’ve created a nice workshop space and intend to get back to more of this stuff. I must say though that it will be a while before I attempt a scratch built Trainwreck! I have followed your threads with interest for some time.

Cheers,
Rob

Keefy

Ossyrocks said:

Keefy said:

You've probably already thought of this, but how old are the filter caps? Might be time to renew them.

I replaced those when I originally built the amp about 10 years ago. I usually use Sprague Atoms, but for some reason at the time I used F&T’s, but I think they’ll still be ok for a while.

With lockdowns and having more time in my hands I’ve created a nice workshop space and intend to get back to more of this stuff. I must say though that it will be a while before I attempt a scratch built Trainwreck! I have followed your threads with interest for some time.

Cheers,
Rob

Thanks! You're basically watching someone, with just enough knowledge to be dangerous, muddling through...

Ossyrocks

Keefy said:

Ossyrocks said:

Keefy said:

You've probably already thought of this, but how old are the filter caps? Might be time to renew them.

I replaced those when I originally built the amp about 10 years ago. I usually use Sprague Atoms, but for some reason at the time I used F&T’s, but I think they’ll still be ok for a while.

With lockdowns and having more time in my hands I’ve created a nice workshop space and intend to get back to more of this stuff. I must say though that it will be a while before I attempt a scratch built Trainwreck! I have followed your threads with interest for some time.

Cheers,
Rob

Thanks! You're basically watching someone, with just enough knowledge to be dangerous, muddling through...

Ha ha, as am I. I know how to not kill myself, after that, it’s all just learning and experimenting. 

Ossyrocks

I’m down a rabbit hole now. I haven’t taken my face out of Rob Robinette’s amp pages for two days. I ordered some parts from @Modulus_Amps on Sunday to crack on with what I thought I wanted to do, but 24 hours later I’ve ordered more stuff from @Modulus_Amps for other mods I want to do. Sorry Modulus, I’ll try not to make an order every day this week.

Rob

PeteC

Ha ! SLO100 or TW express next !! 

Ossyrocks

PeteC said:

Ha ! SLO100 or TW express next !! 

Never say never! 

Some kind of lower wattage Express might be nice. I’ve had a Ceriatone Express and a Fuchs Train 45, but whilst both were great amps, they were very loud. 

Modulus_Amps

Ossyrocks said:

I’m down a rabbit hole now. I haven’t taken my face out of Rob Robinette’s amp pages for two days. I ordered some parts from @Modulus_Amps on Sunday to crack on with what I thought I wanted to do, but 24 hours later I’ve ordered more stuff from @Modulus_Amps for other mods I want to do. Sorry Modulus, I’ll try not to make an order every day this week.

Rob

ha ha,  that will be Chris's problem then, I will let you know if he moans

As for dropping resistors, as a quick test I stuck a 25k linear pot in the position after the phase inverter on one amp and was able to modify the value on the fly to see what it did to the tone, feel and clarity of the amp, its pretty interesting to do , subtle effect - but higher voltages definitely sounded brighter and clearer, which on that amp was the right thing, but for others you may want to shoot for lower voltages, you really need a higher wattage pot than I used for this, but you could put resistors on a switch too.

Lowering the voltage of a gain stage will make it easier to distort that stage, especially if it is bypassed, as well as add more harmonics

RickLucas

However, times have changed and I’ve got a bit more sensible

Ossyrocks

RickLucas said:

However, times have changed and I’ve got a bit more sensible

How are you doing @RickLucas ? One day, when this is all over, you should have a run up to the coast for a brew and a catch up.

Rob

Ossyrocks

Modulus_Amps said:

Ossyrocks said:

I’m down a rabbit hole now. I haven’t taken my face out of Rob Robinette’s amp pages for two days. I ordered some parts from @Modulus_Amps on Sunday to crack on with what I thought I wanted to do, but 24 hours later I’ve ordered more stuff from @Modulus_Amps for other mods I want to do. Sorry Modulus, I’ll try not to make an order every day this week.

Rob

ha ha,  that will be Chris's problem then, I will let you know if he moans

As for dropping resistors, as a quick test I stuck a 25k linear pot in the position after the phase inverter on one amp and was able to modify the value on the fly to see what it did to the tone, feel and clarity of the amp, its pretty interesting to do , subtle effect - but higher voltages definitely sounded brighter and clearer, which on that amp was the right thing, but for others you may want to shoot for lower voltages, you really need a higher wattage pot than I used for this, but you could put resistors on a switch too.

Lowering the voltage of a gain stage will make it easier to distort that stage, especially if it is bypassed, as well as add more harmonics

Today I figured it’s probably best to measure my voltages first! Ha ha

B+1 is 444v.
B+2 is 442v
B+3 is 434v !!!
B+4 is 400v !!!

Whilst the first two are ballpark Bandmaster Reverb (what this is based on) and also silverface Vibrolux Reverb (what I’m aiming at). The second two readings are a lot higher than the Bandmaster schematic with the stock value dropping resisters of 1k and 4.7k respectively. I don’t really understand why they are so much higher than spec, my B+3 and B+4 should be 395v and 360v respectively with the dropping resisters I have as stock.
I may have to experiment here to see how much I can get these down to where they should be.

Thanks for listening,
Rob

Modulus_Amps

Ohms law is your friend, measure the voltage drop acrros each plate resistor to see where the current is going/flowing

Your 1K resistor has 8mA flowing through it (8v drop/1000r)
Your 4.7k has 7.25mA flowing through it 34v/4700r

not looked at the schematic, but to me it doesnt look like you have anything connected to B+3??

Ossyrocks

Modulus_Amps said:

Ohms law is your friend, measure the voltage drop acrros each plate resistor to see where the current is going/flowing

Your 1K resistor has 8mA flowing through it (8v drop/1000r)
Your 4.7k has 7.25mA flowing through it 34v/4700r

not looked at the schematic, but to me it doesnt look like you have anything connected to B+3??

I do. B+3 runs out of the cap can (brown wire) to the phase inverter junction of the two anode load resisters on pins 1 and 6. These are now 82k and 100k as per AB763. 

I’m trying honest guv, but there’s a lot to learn.

PeteC

Did you have any tubes out for those readings.  If so there will be no current draw in that stage and no corresponding voltage drop.  

Ossyrocks

PeteC said:

Did you have any tubes out for those readings.  If so there will be no current draw in that stage and no corresponding voltage drop.  

No, fully loaded and speaker connected. Looking at the various schematics now, the readings I’m getting are ballpark Super Reverb and Vibroverb if not slightly lower. This kind of makes sense given the donor chassis I’m using. I only checked these other schematics just now to see if this was “normal”.