Hi, I've been trying to pinpoint the source of a noise in a 6505+ head. These things are notorious for a bit of "extra hiss" on the lead channel, but this is a roar like a waterfall. With the gain up to 75% or so, and the master just barely turned past 2, it's so loud that it's unplayable. It's just as noisy with no instrument connected. I tried replacing all input tubes one at a time, and I noticed that upon pulling V1 out, the amp is nearly silent, even with the master all the way up. I tired running it with the input stage plate resistor disconnected, and it was a little quieter, but still too much. I noticed that the first gain stage of the lead channel has a 470K series grid resistor (stopper?). Replacing it with a new metal film didn't quiet anything down either. Then I replaced every series resistor in the entire front end (input and 1st and 2nd gain stages of both channels) with new metal films. No change in noise. I am presuming that the original amp was at least playable, so I am still looking for a fault somewhere before I start trying to modify something. I looked at all solder joints under a microscope and there is no evidence of cracking or cold joints. Any advice would be wonderfully appreciated!
Input jacks dont Hiss.
It is amplifier design.
Gain after Gain after gain for lead channel.
Main issue is " effect loop"
Brings that gain all way way way down for " line level"
Then another gain stage to bring it alll back up again.
So the compound noise is very high.
So yes with even early low gain tube heads.
V1 tend to be a very high gain stage regardless.
So a noisy tube there will carry out rather guickly.
With a 6505 even my boogie rec.
Its almost every stage is high gain.
So any hissy tubes get greatly boosted anywhere.
I basically used my dual rec as a " tube tester"
if any pre tube sounds vaguely more quiet in a noise
blaster dual rec. It will be very guiet in a normal amp lol.
Your looking for the closest renamed modern gimmick
tube that resembles a 7025 low noise tube.
This is a 12AX7 with small plates and then spiral wound
heaters. there is one company pushing " spiral wound"
magic heaters but it is larger plate 12AX7.
Your basically looking for lowest noise basic everyday
" guitar" small plate 12AX7.
As you found V1 is a big contributor.
Another contributor is figure out, is the tube that
brings all the gain back after the effects loop.
That one will have a buttload of gain too towards the end.
It is amplifier design.
Gain after Gain after gain for lead channel.
Main issue is " effect loop"
Brings that gain all way way way down for " line level"
Then another gain stage to bring it alll back up again.
So the compound noise is very high.
So yes with even early low gain tube heads.
V1 tend to be a very high gain stage regardless.
So a noisy tube there will carry out rather guickly.
With a 6505 even my boogie rec.
Its almost every stage is high gain.
So any hissy tubes get greatly boosted anywhere.
I basically used my dual rec as a " tube tester"
if any pre tube sounds vaguely more quiet in a noise
blaster dual rec. It will be very guiet in a normal amp lol.
Your looking for the closest renamed modern gimmick
tube that resembles a 7025 low noise tube.
This is a 12AX7 with small plates and then spiral wound
heaters. there is one company pushing " spiral wound"
magic heaters but it is larger plate 12AX7.
Your basically looking for lowest noise basic everyday
" guitar" small plate 12AX7.
As you found V1 is a big contributor.
Another contributor is figure out, is the tube that
brings all the gain back after the effects loop.
That one will have a buttload of gain too towards the end.
That's a very large series resistance, going to create a whole lot of Johnson noise. Grid stoppers don't need to be anything like that value, try 1k to 10k range perhaps?
In a sensible design most of the noise is from the input stage only, since it has enough gain to put the signal out of reach of noise later on.
Careful!
If that 470k is capacitor-coupled from a previous gain stage, reducing it to 10k will make blocking distortion ("farting out") more likely.
[ETA: I dont have the schematic to hand to check this.]
Also, it will increase treble response: that resistor also forms a high frequency roll-off in conjunction with the tube's Miller capacitance
Cheers, and regards,
Ant
If that 470k is capacitor-coupled from a previous gain stage, reducing it to 10k will make blocking distortion ("farting out") more likely.
[ETA: I dont have the schematic to hand to check this.]
Also, it will increase treble response: that resistor also forms a high frequency roll-off in conjunction with the tube's Miller capacitance
Cheers, and regards,
Ant
Thanks everyone, I appreciate the help!
It is indeed capacitor coupled. I wanted to be careful about lowering that series resistance for all those reasons mentioned plus the assumption that this thing was at least useably quiet when new.
My review of my Blencowe/Kuehnel/Jones books remind me that early stages MUST be kept clean, and it's probably a series resistor (johnson) or an early gain stage tube causing the noise. I suspect having the input stage plus Lead stage 1 on the same tube was to contain it a known quiet tube to handle the initial gain.
I got zero change with AX7/(moderately used)7025 tube swaps, even an AU7 made little difference. I've replaced all series resistors in the first couple gain stages with good quality metal film, with zero change.
Pulling V1 silences the amp. This makes me think it MUST be input or first lead gain stage. When I lift the input tube's plate resistor, it's better, but still noisy.
Lead stage 1 is fed from the input stage through a 1M Pot to ground(Lead Pre control) paralleled with a couple of 1M resistors. On low volume the lead pot would short the grid to ground without that 470k resistor. There is no explicit grid leak resistor, just 470k plus Lead pot...
My confusion is what could have CHANGED to cause this much noise. Should I suspect the Lead pre pot?
It is indeed capacitor coupled. I wanted to be careful about lowering that series resistance for all those reasons mentioned plus the assumption that this thing was at least useably quiet when new.
My review of my Blencowe/Kuehnel/Jones books remind me that early stages MUST be kept clean, and it's probably a series resistor (johnson) or an early gain stage tube causing the noise. I suspect having the input stage plus Lead stage 1 on the same tube was to contain it a known quiet tube to handle the initial gain.
I got zero change with AX7/(moderately used)7025 tube swaps, even an AU7 made little difference. I've replaced all series resistors in the first couple gain stages with good quality metal film, with zero change.
Pulling V1 silences the amp. This makes me think it MUST be input or first lead gain stage. When I lift the input tube's plate resistor, it's better, but still noisy.
Lead stage 1 is fed from the input stage through a 1M Pot to ground(Lead Pre control) paralleled with a couple of 1M resistors. On low volume the lead pot would short the grid to ground without that 470k resistor. There is no explicit grid leak resistor, just 470k plus Lead pot...
My confusion is what could have CHANGED to cause this much noise. Should I suspect the Lead pre pot?