Alternate PPI MV idea

kannibul · Post by **kannibul** » Tue Sep 20, 2005 11:34 pm

Granted, nothing is new, so I'm probably re-hashing someone else's idea

I'm proposing, on my Metro Superlead kit:

1) Remove the Bias Splitter resistors
2) Install a dual 250K Pot
3) At the junction point between where bias splitter resistors were located, a lead will be ran to both of the the 3rd terminals on the sub-pots.
4) The caps that went to join with the Bias Splitter resisitors, and also would connect with the leads to the power tube pairs, would be lifted from the turret, and have leads ran to the wiper on each sub-pot.
5) The turret that is connected to the power tubes, would have a lead running to each of the 1st lug on the sub-pots.

The goal: Have the pot act as the bias splitter resistors, so that it doesn't mess with the bias when the pot's value changes. If the wiper fails, no possible damage could occur to the amp.

Anyone do this? If so, what did it sound like over the "rich mod"?

Also, I claim ignorance if anything on here sounds stupid! Just tell me it's the dumbest thing you have ever heard, and I'll scrap the idea, and I'll thank you for saving me time and money

Thanks!

Billy Batz · Post by **Billy Batz** » Wed Sep 21, 2005 6:43 am

What your describing already happens in the Rich mod. The Rich mod is safer because it has the existing splitter resistors accross the variable pot resistance so the entire bias voltage isnt on the pot tracks. If you adjust the value of the splitter resistors that are fixed then the rich mod should act exactly the same as your idea only the two fixed resistances will make sure the bias voltage will always see a resistance in case the pot tracks get flakey. Change the fixed bias splitter value to 400k because 400k in parallel with the 500k resistance of the pot (may vary a bit) will give you 222k at full volume with the pot. Then as you turn it down it will go to 0 ohms so it works just as well. The rich also puts a resistance in line with the AC signal after the output couplers. You can use a set of small jumper cables to jump the two right terminals of each of the dual pots to take that resistance away and become exactly what your talking about. I imagine that resistance is there because the sound gets very bright as the volume gets lower.

kannibul · Post by **kannibul** » Wed Sep 21, 2005 9:33 am

Billy Batz wrote:What your describing already happens in the Rich mod. The Rich mod is safer because it has the existing splitter resistors accross the variable pot resistance so the entire bias voltage isnt on the pot tracks. If you adjust the value of the splitter resistors that are fixed then the rich mod should act exactly the same as your idea only the two fixed resistances will make sure the bias voltage will always see a resistance in case the pot tracks get flakey. Change the fixed bias splitter value to 400k because 400k in parallel with the 500k resistance of the pot (may vary a bit) will give you 222k at full volume with the pot. Then as you turn it down it will go to 0 ohms so it works just as well. The rich also puts a resistance in line with the AC signal after the output couplers. You can use a set of small jumper cables to jump the two right terminals of each of the dual pots to take that resistance away and become exactly what your talking about. I imagine that resistance is there because the sound gets very bright as the volume gets lower.

The thing that concerns me, is that with the volume control at 0, the bias splitter resistors are essentially at 0. Doesn't this make a difference?

Concern is too strong of a word, but it's the only one I can think of at the moment!

Also, would the imbalance of the pots make a difference?

I know that the one I have is off by about 30-40ohms from the other.

kannibul · Post by **kannibul** » Wed Sep 21, 2005 10:08 pm

flemingmras · Post by **flemingmras** » Thu Sep 22, 2005 1:43 am

Actually, the bias splitter resistors are actually known as the power tube grid load resistors. The higher the value the more gain you get from the power tubes. The grid load resistance has to be there for the tube to work properly.

These resistors are not at 0 ohms with the pot at 0, rather the grids of the power tubes see 0 ohms on it's grid with respect to the bias supply. With no grid load resistance, you will not get any output from the power tubes, which is what's supposed to happen when the PPIMV pot is at 0.

This will not hurt anything at all.

Jon

kannibul · Post by **kannibul** » Thu Sep 22, 2005 8:51 pm

flemingmras wrote:Actually, the bias splitter resistors are actually known as the power tube grid load resistors. The higher the value the more gain you get from the power tubes. The grid load resistance has to be there for the tube to work properly.

These resistors are not at 0 ohms with the pot at 0, rather the grids of the power tubes see 0 ohms on it's grid with respect to the bias supply. With no grid load resistance, you will not get any output from the power tubes, which is what's supposed to happen when the PPIMV pot is at 0.

This will not hurt anything at all.

Jon

thanks!

Where can I find great information like this? Any recommended reading?

(Not to take away from Billy's post - thanks to you too!)

Bainzy · Post by **Bainzy** » Fri Sep 23, 2005 9:52 am

flemingmras wrote:Actually, the bias splitter resistors are actually known as the power tube grid load resistors. The higher the value the more gain you get from the power tubes. The grid load resistance has to be there for the tube to work properly

if that's the case, couldnt you just increase the resistor values really high to get a high gain marshall sound from the power amp?

Dai H. · Post by **Dai H.** » Fri Sep 23, 2005 10:33 am

no there is a limit to how high. Take a look at the datasheets.

Billy Batz · Post by **Billy Batz** » Fri Sep 23, 2005 11:36 am

You also run the risk of oscillation and other noises.

kannibul · Post by **kannibul** » Fri Sep 23, 2005 2:19 pm

Bainzy wrote:
flemingmras wrote:Actually, the bias splitter resistors are actually known as the power tube grid load resistors. The higher the value the more gain you get from the power tubes. The grid load resistance has to be there for the tube to work properly
if that's the case, couldnt you just increase the resistor values really high to get a high gain marshall sound from the power amp?

From what I've read, you lower those resistors to get more grind out of the amp.

http://marshall.redpt.com/clay/marshall ... s_101.html

Also the 70's Marshalls saw different bias splitter values. 82K, 100K, 150K and the common 220K. The smaller values will load down the preamp and roll off the top end. The 82K/100K was usually used when 6550's came stock in the amp. A lot of people had the 6550's switched over to EL34's and most techs didn't change the bias splitters which resulted in a little more preamp crunch and a "browner" sound to the top end. The 220K's are the classic value for use with EL34's.

Maybe I got it backwards?

flemingmras · Post by **flemingmras** » Fri Sep 23, 2005 3:29 pm

Uh yeah...lowering the grid load resistors on the power tubes drops the input impedance of the power stage, which loads down the phase inverter, resulting in high end rolloff and a browner sound. And yes they will result in more grind...FROM THE PREAMP!!! NOT THE POWER AMP!!!

Remember this...gain=headroom. However, when all that gain is BEFORE everything you're gonna run out of headroom in the other stages of the amp. Having high gain on the power section gives more headroom...however if you push it really hard with high gain the power stage will clip.

Jon

kannibul · Post by **kannibul** » Fri Sep 23, 2005 9:35 pm

flemingmras wrote:Uh yeah...lowering the grid load resistors on the power tubes drops the input impedance of the power stage, which loads down the phase inverter, resulting in high end rolloff and a browner sound. And yes they will result in more grind...FROM THE PREAMP!!! NOT THE POWER AMP!!!

Remember this...gain=headroom. However, when all that gain is BEFORE everything you're gonna run out of headroom in the other stages of the amp. Having high gain on the power section gives more headroom...however if you push it really hard with high gain the power stage will clip.

Jon

Is there a tube-amps for dummies book anywhere?

I do understand what your saying though, I think. I really need to learn more - I'm hungry for it, but I don't know where to start.

flemingmras · Post by **flemingmras** » Sat Sep 24, 2005 1:19 am

Get the book "Inside Tube Amps" by Dan Torres.

Also...you may want to learn basic electronics first so that you have an understanding of what different components and circuits do just by looking at them.

Email me at wildercyde@comcast.net and I can get you started.

Jon

Billy Batz · Post by **Billy Batz** » Sat Sep 24, 2005 7:16 am

The Kevin OcConnor series is great if your already knowledgable about general electronics and know how to read schematics. Or if you can learn really well yourself.

Dai H. · Post by **Dai H.** » Sat Sep 24, 2005 8:12 am

http://www.aikenamps.com/TechInfo.htm

is good. Various articles on the web like stuff in AGA, the web pages such as Lord Valve's bias FAQ--good instructional on biasing:

http://aga.rru.com/

lots of old tube book scans, etc.:

http://www.pmillett.com/

should be tons of stuff if you google.