Tomorrow I will finally be able to modify the amp.
Regarding brightness I shall
- drop grid stoppers to 10k
- add 100pf Silver Mica to volume pot
- drop 1st cathode bypass cap to 0.68uF Wima Polypropylene
- change coupling caps from 0.022uF PIOs to 0.01uF Mallory 150's
That should help without cutting all the bass but enhance highs and remove the lowest lows.
Other mods
- remove 6n1p and replace with 12ax7
- change nfb resistor from 22k to 10k and add a spst switch and 100k pot in series with the nfb tap, so the nfb goes from 10k to 100k and none.
Any ideas to do with the low input jack, I've never found any use to it!?
old thread
Moderators: pompeiisneaks, Colossal
Re: old thread
Still no soldering iron.
I went through all the caps that are going into the amp to check the outer foils for correct orientation. On Mallory's it was very clear, Silver Micas apparently don't have one and Wima also didn't seem to have, maybe because it's such a high voltage cap.
I am changing the low input to a high input bright channel like in some Supros, by adding a 4.7nF cap in series just before the grid stoppers.
What is the correct orientation for this cap?
I went through all the caps that are going into the amp to check the outer foils for correct orientation. On Mallory's it was very clear, Silver Micas apparently don't have one and Wima also didn't seem to have, maybe because it's such a high voltage cap.
I am changing the low input to a high input bright channel like in some Supros, by adding a 4.7nF cap in series just before the grid stoppers.
What is the correct orientation for this cap?
Re: old thread
remember that you need a ground reference for preamp grids, and a voltage reference for power amp grids too.utervo wrote:by adding a 4.7nF cap in series just before the grid stoppers.
Re: old thread
What do you mean?roberto wrote:remember that you need a ground reference for preamp grids, and a voltage reference for power amp grids too.utervo wrote:by adding a 4.7nF cap in series just before the grid stoppers.
I was planning to change the low input to be like the hi input including the 1 Mohm in the circuit, and add a cap between 1 Mohm and grid stopper to cut bass.
Re: old thread
Ok I think you mean the 1 Mohm resistor, yes I will add that there too, thanks.
Re: old thread
Hi
This has been a funny thread on a Fender Champ. There has been some bad info too. A while ago I put five different 0.022uF caps on a switch to hear the difference. To ensure playing wasn't a variable I played recorded info into the amp, there were no tonal differences at all.
Casey Jones talked about getting rid of the 8uF caps. This can be bad info too. These caps provide compression within the amp. I'm currently using 47uF caps with a 6L6 and the amp is quiet, but there isn't a lot of compression.
I'd agree transformers and speakers are important. I like the Weber Silver Ten as the bass response is good for this amp.
Shielded cable is a must and I like a PI filter prior to the output supply node to reduce hum.
BTW the Gibson GA-5 is a nice sounding amp and worth a look too.
My two cents.
This has been a funny thread on a Fender Champ. There has been some bad info too. A while ago I put five different 0.022uF caps on a switch to hear the difference. To ensure playing wasn't a variable I played recorded info into the amp, there were no tonal differences at all.
Casey Jones talked about getting rid of the 8uF caps. This can be bad info too. These caps provide compression within the amp. I'm currently using 47uF caps with a 6L6 and the amp is quiet, but there isn't a lot of compression.
I'd agree transformers and speakers are important. I like the Weber Silver Ten as the bass response is good for this amp.
Shielded cable is a must and I like a PI filter prior to the output supply node to reduce hum.
BTW the Gibson GA-5 is a nice sounding amp and worth a look too.
My two cents.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: old thread
I have three 22uf F+T's now, seem ok but don't really know, very sensitive to touch, a very unfordigin amp atm!Mark wrote: Casey Jones talked about getting rid of the 8uF caps. This can be bad info too. These caps provide compression within the amp. I'm currently using 47uF caps with a 6L6 and the amp is quiet, but there isn't a lot of compression.
Which position do you exactly mean for an extra filter cap?Mark wrote: Shielded cable is a must and I like a PI filter prior to the output supply node to reduce hum.
Can anyone advise my with the voltages, when I measured them which is a while ago (live 4 years ago)...
1st filter cap: 362V
2nd filter cap: 318V
3rd filter cap: 258V
I understand the the 258v is to preamp tube, but the two other are for the poweramp tube but which is the voltage that everyone always talks as the voltage for poweramp?
Either 362V and 318V seem very high for 6AQ5A, how do I lower them without lowering the preamp voltages which seem inline?
Maybe I should just change it to 6V6GT.
Re: old thread
Can I answer everything a bit later?
http://www.ax84.com/static/corepoweramp ... ematic.pdf
The link shows the circuit I'm talking about. After the standby switch there is a 47uF filter cap followed by R1 a 130 ohm 5 watt resistor and another 47uF filter cap. Notice this node isn't used in the power stage at all. This is the preceding PI filter (the name is derived from the fact it resembles the letter PI with the two caps being vertical while the resistor is horizontal) will drop your rail voltage by 30 to 40 volts. You'll kill two birds with one stone!
I hope this helps.
http://www.ax84.com/static/corepoweramp ... ematic.pdf
The link shows the circuit I'm talking about. After the standby switch there is a 47uF filter cap followed by R1 a 130 ohm 5 watt resistor and another 47uF filter cap. Notice this node isn't used in the power stage at all. This is the preceding PI filter (the name is derived from the fact it resembles the letter PI with the two caps being vertical while the resistor is horizontal) will drop your rail voltage by 30 to 40 volts. You'll kill two birds with one stone!
I hope this helps.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: old thread
Thanks Mark!
I probably can't use 47uF, 5Y3 can't take it. Can I just add a resistor to drop voltage?
Don't the reservoir capacitors before used nodes affect "feel", is it only the ones at the nodes to power tube screen and preamp tube that have the effect?
I probably can't use 47uF, 5Y3 can't take it. Can I just add a resistor to drop voltage?
Don't the reservoir capacitors before used nodes affect "feel", is it only the ones at the nodes to power tube screen and preamp tube that have the effect?
Re: old thread
33uF should be good enough for a 5Y3.
Also consider not using a valve rectifier at all. A true class A amp does have any sag at all and in fact the rail voltage rises when signal is applied (seen it on my CRO).
The sag in a Champ comes in when the screen current kicks in and lowers the preamp voltage rail, this results in less gain.
Also consider not using a valve rectifier at all. A true class A amp does have any sag at all and in fact the rail voltage rises when signal is applied (seen it on my CRO).
The sag in a Champ comes in when the screen current kicks in and lowers the preamp voltage rail, this results in less gain.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: old thread
Just measured the voltages again.
The power supply voltages were 366V, 315V and 253V all measured to ground. Cathode to ground was 20V, so is my plate and screen grid voltages actually 346V and 295V?
Measured preamp plates were 105V and 115V, so is the actual voltages 253-105 and 253-115, so 148V and 138V? Cathode to ground on V1 was 2V.
Maybe I should just drill a bigger hole and swap to a real 6V6 instead of trying to shave off that much voltage? 6AQ5A is rated for 250V and some 275V.
The power supply voltages were 366V, 315V and 253V all measured to ground. Cathode to ground was 20V, so is my plate and screen grid voltages actually 346V and 295V?
Measured preamp plates were 105V and 115V, so is the actual voltages 253-105 and 253-115, so 148V and 138V? Cathode to ground on V1 was 2V.
Maybe I should just drill a bigger hole and swap to a real 6V6 instead of trying to shave off that much voltage? 6AQ5A is rated for 250V and some 275V.
Re: old thread
Can I ask you why you are so concerned about voltages?
Are voltages causing you issues such as shortened tube life?
What circuit are you using or rather how does it differ from the stock Champ circuit?
Some of these Champ amps run pretty high voltages. I suspect the voltages you mention aren't too different from what you'd expect in a Tweed Champ.
My Champ clone uses a 5AR4 and has a B+ of 400VDC, and uses a 6L6. If you have an output transformer with a 5K primary, this isn't a bad option as the amp should put out 8 to 10 watts. The amp will sound louder, the capability of the power transformer will be the defining factor.
Are voltages causing you issues such as shortened tube life?
What circuit are you using or rather how does it differ from the stock Champ circuit?
Some of these Champ amps run pretty high voltages. I suspect the voltages you mention aren't too different from what you'd expect in a Tweed Champ.
My Champ clone uses a 5AR4 and has a B+ of 400VDC, and uses a 6L6. If you have an output transformer with a 5K primary, this isn't a bad option as the amp should put out 8 to 10 watts. The amp will sound louder, the capability of the power transformer will be the defining factor.
Yours Sincerely
Mark Abbott
Mark Abbott
Re: old thread
Only because of the 6AQ5A I have instead of 6V6GT!Mark wrote:Can I ask you why you are so concerned about voltages?
Are voltages causing you issues such as shortened tube life?
What circuit are you using or rather how does it differ from the stock Champ circuit?
Re: old thread
Decided to order a VVR to reduce B+ if needed...
Re: old thread
Lots of work here for a Champ. Good thing you're not building a Deluxe Reverb 