Switching channels vs mixing channels, mixing resistor

[shocki]

Right now I am doing a Blackface / Trainwreck modification to my Bassman. Schematic looks like this http://www.billyz.net/Fender%20Bassman% ... 20mod.html

Both channles are mixed via 220K resistors. This has the advantage I can mix those channels. But it creates quite a lot of hiss since both channels are active all the time. So I though I could put a switch at the point where we have the mixer resistors right now and remove those.

Another option would be to put a switch directly after the first tube and just use one input? And probably it would already reduce the hiss quite a lot.

I think this should remove a lot of noise. But will also change the sound.
As far as I understand the function of the mixing resistor is to block the signal going back into the other amp input. It should also function as a grid stopper? Since there is still some current flowing through this resistors it should decrease the gain a bit and works like a vol control. Is this correct?

So a bigger mixer resistor has less gain? So when I remove those resistors I will have more gain and I removed a grid stopper which would change frequency responsive?

Question could also be how do I make single channel amp out of this 2 channel bassman? I have seen people leaving in the both 220K and just ground one of those to have the same effect as the had with 2 channels.

If it works like I think it does it would have more gain. And I am not sure if this a good thing for the "blackface" vibe. I want to keep the normal channel really Fender clean.

Thanks alot for your help.

[Stevem]

Note that you can not as you say mix those channels, as the Bass channel has one more gain stage in it and as such is out of phase with the normal channel.
The end result of doing that with both channels being used will be having the guitar sound like the middle pickup position on a Strat, which. Is out of phase.
You can only use one channel at a time.

[shocki]

Note that you can not as you say mix those channels, as the Bass channel has one more gain stage in it and as such is out of phase with the normal channel.

The end result of doing that with both channels being used will be having the guitar sound like the middle pickup position on a Strat, which. Is out of phase.
You can only use one channel at a time.

Thanks. But this is Ok with me.

[Stevem]

In that case if you just want to use a simple switch to choose the number of channels go with a DPDT type switch installed before the 220k mixers and keep there 220k value.
In regards to noise you are getting, if its hiss than that is made by gain and or noisy resistors, however if the sound you are talking about sounds like ocean waves or wind thru a somewhat open car window, than that is caused by a bad preamp tube!
when you mod the amp change out all the plate load resistors, cathode resistors and grid resistors, and also the 68k resistors off of the input jacks.
note that the all the plate load resistors should be upped to 1 waters, and all the others are fine at the socoker 1/2 watt rating.
Another nice cheap mode to make for a more pleasing sound is to replace the 7 power supply restos with fast recovery UF5408, or UF5406 types.
You only need 4 of the 5408s in the main B+ supply, down from the original 6 and then one for the bias supply recto.
The leads on the 5408s are big so you will have to rap one lead around another and use one recto lead for the connection back into the solder hole grommet.
the use of these type diodes makes for less noise hash in the power supply voltage which we are so use to hearing that when its gone you wonder why in amps that use diodes that more people do not make this change!

another mod you should make if the amp will be played up over half volume is to increase the UF value of the first two stacked filter to 220uf types at 350 volts, and also check that the 100 ohm resistors off of the pilot lamp (filament supply) to ground are not burned up and or gone completely!

[shocki]

Thanks alot so far.

In that case if you just want to use a simple switch to choose the number of channels go with a DPDT type switch installed before the 220k mixers and keep there 220k value.

Can you tell where to place this exactly and why I should keep the 220k resistors? Like in the schematic I attached? I still would very interested understanding what the mixer resistors actually do to the sound. Is what I desribed above correct or not?

In regards to noise you are getting, if its hiss than that is made by gain and or noisy resistors, however if the sound you are talking about sounds like ocean waves or wind thru a somewhat open car window, than that is caused by a bad preamp tube!

Preamp tubes are OK. I already swapped the plate resistors and mixing resistors for carbon film. The noise is much better now. If just use the normal channel (pulling both tubes of bass channel) the noise is OK. If I add the tubes to the bass channel (which has more gain) it adds noise. This is why I want to the bass channel out of the circuit when I just use the normal channel. I will try and replace the cathode and grid resistors also. Also I did not do the input jacks yet.

Another nice cheap mode to make for a more pleasing sound is to replace the 7 power supply restos with fast recovery UF5408, or UF5406 types.
You only need 4 of the 5408s in the main B+ supply, down from the original 6 and then one for the bias supply recto.
The leads on the 5408s are big so you will have to rap one lead around another and use one recto lead for the connection back into the solder hole grommet.
the use of these type diodes makes for less noise hash in the power supply voltage which we are so use to hearing that when its gone you wonder why in amps that use diodes that more people do not make this change!

I never heard about this and don't know the theoretical background behind it. But is sounds good. Is this cause the 5408 have a higher voltage rating? So I just need 4 of those?

another mod you should make if the amp will be played up over half volume is to increase the UF value of the first two stacked filter to 220uf types at 350 volts, and also check that the 100 ohm resistors off of the pilot lamp (filament supply) to ground are not burned up and or gone completely!

220uf would give me less hum and tighter bass. Right?

[pops]

The shorting jacks on the input of the bass channel should stop all sound and noise if nothing is plugged into them. Check to see that they are indeed shorting the signal to ground at the input. Should be no need to remove the tubes from the bass channel.

[billyz]

On a few of these ( for folk with sensitive ears) I have used 500pf over the 50k plate load on v1. I used 500pf because it was already there , removed from the normal channel. I have found some reduction in hiss by replacing the 68k the 820 cathode , and the plate load resistors with my favorite , IRC 1/2w rn series , also the Dale rn series . These are getting harder to find so I have been using PRP. Usually I use two 1/2w 100k to get the 50k plate load. Lead dress is important. Tubes make a huge difference, follow standard Trainwreck express procedure to find the quietest.
If you were to use a switch to select channels then you might not even want the 220k mix resistors, and add some fidelity.
Z

[shocki]

The shorting jacks on the input of the bass channel should stop all sound and noise if nothing is plugged into them. Check to see that they are indeed shorting the signal to ground at the input. Should be no need to remove the tubes from the bass channel.

Mhh. There is no difference when i remove the cable from the bass input. The noise does not get less.

If you were to use a switch to select channels then you might not even want the 220k mix resistors, and add some fidelity.

That is what I tought. Those cut of some treble since they also act as grid blockers? Do they actually decrease the signal level?

[Leo_Gnardo]

Note that you can not as you say mix those channels, as the Bass channel has one more gain stage in it and as such is out of phase with the normal channel.

The end result of doing that with both channels being used will be having the guitar sound like the middle pickup position on a Strat, which. Is out of phase.
You can only use one channel at a time.

Thanks. But this is Ok with me.

And you could always add a triode plate-follwer configured with R's for unity-gain as a phazer phlipper. Or even a signal transformer if you dare.

Should you go switch and have no mix, I'm with BillyZ - leave out "mix" resistors. Apply stopper resistors at the tube socket, and only as necessary.

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Power supply rectifiers - I've used UF4007 plenty of times, some small reduction in buzz. Havent tried the UF54xx series but seems like overkill with their 3A rating vs 4007's 1A. Also tried FREDs, a bit expensive, definitely overkill, couldn't hear any advantage over UF4007 in Fender/Marshall style amps.

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Stopper resistor at input - need not be the "traditional" 68K. I've tried "low noise" metal films, not really much of a reduction in noise. CC resistor placed at the tube socket, and use the lowest value that gives you stability. CC are recommended by the audiophiles-who-know because their resistance is constant up into the RF region, unlike spiral-cut MF, MOX and CF types which means they are inductors plus resistors at hi frequencies.

[billyz]

The shorting jacks on the input of the bass channel should stop all sound and noise if nothing is plugged into them. Check to see that they are indeed shorting the signal to ground at the input. Should be no need to remove the tubes from the bass channel.

Mhh. There is no difference when i remove the cable from the bass input. The noise does not get less.

Check the shorting jack on the bass channel. I insert a jack and tweak the shorting contact with thin needle nose to insure good contact when the plug is removed. May also slip some 600 sandpaper lubed with deoxit between the contact to clean it.

[Mant]

But out of phase is out of phase.
If you are combining youbarent getting the best of what each channel has to offer.
It is subtractive.
One will be pushing while the other is pulling and where they match they will cancel.
It osnt a smart way to go.
Im not saying thsi because its not technically correct.
Im saying it because it really doesnt sound good.
The low end gets weird and starts cancelling and it gets comb filtery
Thru tye mids.
Its confused.
So you are gonna go thru all that effort only to combine it incorrectly.
Obviously you will do what you want and this isnt meant as a flame in any way. I have spent most f my life in a recording studio and this is the kind of stuff that will drive you crazy.
Anyways i said it and i mean it for the best.
Good luck

[shocki]

If you are combining youbarent getting the best of what each channel has to offer.
It is subtractive.

Of course you are right. But I will not combine the channels. I will use either the blackface or the Trainwreck channel.

[billyz]

If you are combining youbarent getting the best of what each channel has to offer.
It is subtractive.

Of course you are right. But I will not combine the channels. I will use either the blackface or the Trainwreck channel.

I have used both channels in this way. plug into the bass channel input 1 , then come out of the bass ch 2 into an effects pedal( I like the wampler Spring reverb) then into the nornal ch. you now have full control over the effect. And it may or may not be in phase.