Reducing hiss in an amp.

[Mike_J]

Was reading Merlin who regards the 68K grid stoppers as introducing the most hiss into an amp of any resistors. He recommends a 10k resistor together with a 100pF to 470pF capacitor from grid to ground to reduce hiss.

Have designed my amp to have only one grounding point which is near one of the input jacks. The input jacks have been isolated. RG316 cable was used as the grid wire to the 68K grid stoppers. They are connected at a modified vertical terminal strip. The shield for the cable is grounded at only the input jack ground lug.

Presuming I am going to use Merlin's system of connecting a capacitor from grid to ground could I be introducing a ground loop by grounding the conductor? The shield is grounded at the input jack which is good because I want only one ground point. Was wondering if I could use the shielding on the cable to ground the caps from the tube grid connection? It would be doing double duty and since it would be grounded at only one place then I am not sure where the ground loop would be. Little confused on this one and could use some help.

Thanks
Mike

[terminalgs]

a good place would be the ground of the triode(pentode?)'s cathode resistor. 


the shield of the coax can be connected to any ground.  it is not going to see any current from circuit DC or instrument signal AC as you are connecting the shield on only one side, so any ground that is convenient at either end of the coax works.

[thetragichero]

only ground one end of the sheild because the shield and signal wires are in parallel with an insulator in between them. if that sounds like something familiar it's because a capacitor is two parallel plates with an insulator in between them

[Mike_J]

a good place would be the ground of the triode(pentode?)'s cathode resistor. 


the shield of the coax can be connected to any ground.  it is not going to see any current from circuit DC or instrument signal AC as you are connecting the shield on only one side, so any ground that is convenient at either end of the coax works.

What I was wondering is could I bring a ground wire out of both ends of the cable of course connected to the shielding. On the input side I would connect the shielding to an input jack ground lug thereby making the shield a shield. On the tube socket side I would connect the silver mica to the shield since it is going to the ground I want to use anyway if that makes sense. I wouldn't use it as a shield but as a wire to connect the cap from the grid pin to ground. It would make it easier because I could just use one of the cables coming from the input jack that is connected to a vertical terminal strip and just borrow it to send it back to that input jacks ground lug which would then eventually go to chassis ground near the input jacks. Basically the question is does the cap between the shield and the grid lug on the tube socket break the ground loop? Otherwise grounding both ends of a cable would certainly create a ground loop.


Thanks
Mike

[Mike_J]

a good place would be the ground of the triode(pentode?)'s cathode resistor. 


the shield of the coax can be connected to any ground.  it is not going to see any current from circuit DC or instrument signal AC as you are connecting the shield on only one side, so any ground that is convenient at either end of the coax works.

Thank you for your response. That would certainly be a good alternative to getting the cap grounded.


Thanks
Mike

[Mike_J]

Have attached a picture of the RG316 cables going to their respective lugs on a vertical terminal strip. The 68K grid stoppers are attached to the conductor of the cable. The shield was grounded to the input jack ground lug. There are a total of four cables, two for the normal channel (one hi and one lo) and two the same way for the bright channel. The grid stopper resistors for each pair from the channels are twisted and one wire goes to the grid socket either pin 2 or 7 depending on which channel it is serving.


What Merlin is saying is if I reduce the 68K resistors to 10K resistors and run a cap of a yet to be determined value from the pin or wrap another wire around where the wires are already wrapped to ground then there should be less hiss. Looking at the picture the cathode resistor grounds are looking more attractive. Would just need to put some Teflon coating from a wire on the cap leads so they don't touch anything they shouldn't. Definitely something I will try if I hear much hiss.


Thanks
Mike

[Mike_J]

The reason this concerns me at all is the Hoffman layout has a single input jack sending a cable to a 33K resistor. Whereas, my chassis has four input jacks that are sending a cable with four 68K resistors. While most would probably think it is premature to be concerned about this because I still don't know whether there will be any hiss in my amp at all, I like to plan ahead. If it isn't this amp maybe it will be the next one. Or it could be someone who believes using 1/4 watt carbon film resistors is the way to go as the Bassman reissue schematic is calling for.

[DuaneOh]

The bassman circuit has a hi and lo input. The hi input puts both of the grid stoppers resistors in parallel with 1 meg grid leak to ground, that is why they use 33k with a single input. If you use 10k grid stoppers the hi input will be seeing 5k to the grid. The lo input goes through 1 grid stopper with the other grid stopper to ground, so you reduce your input signal by about half. The lo input with 10k grid stoppers have a about 20k input load instead of about 136k with 68k grid stoppers, that's quite a load for a guitar pickup.

I think it's easy to attach your cap from the grid pin to the cathode pin on the socket since all you are doing is adding to the miller capacitance.

As said previously, only attach one end of the shield on the cable to ground, you do not want current to flow through it. I would attach the ground lug of your input jack to your cathode resistor ground point.

Carbon comp resistors have that "mojo" (?), whether it is something you'll hear or not is up to you. CC resistors are noisier than metal film but you may not hear that difference either.

[Mike_J]

The bassman circuit has a hi and lo input. The hi input puts both of the grid stoppers resistors in parallel with 1 meg grid leak to ground, that is why they use 33k with a single input. If you use 10k grid stoppers the hi input will be seeing 5k to the grid. The lo input goes through 1 grid stopper with the other grid stopper to ground, so you reduce your input signal by about half. The lo input with 10k grid stoppers have a about 20k input load instead of about 136k with 68k grid stoppers, that's quite a load for a guitar pickup.

I think it's easy to attach your cap from the grid pin to the cathode pin on the socket since all you are doing is adding to the miller capacitance.

As said previously, only attach one end of the shield on the cable to ground, you do not want current to flow through it. I would attach the ground lug of your input jack to your cathode resistor ground point.

Carbon comp resistors have that "mojo" (?), whether it is something you'll hear or not is up to you. CC resistors are noisier than metal film but you may not hear that difference either.

Are you saying by lowering the grid stoppers to 10K it will significantly reduce the load on the guitar pickup. That would seem to be a benefit to Merlin's idea as well. Don't know how much of a drain on highs the pF cap to ground would cause. Probably the equivalent of five or ten feet of guitar cord is my guess. Thanks for the don't want current to flow through the shield. That is the point I was uncertain of but looking for. Funny thing, I was reading an article about carbon comps a little while ago and they claimed they added sweetness to the tone if used as plate load resistors for the first few preamp tubes. Apparently not useful in the PI position if the amp has negative feedback according to this author.


I want to test 110K plate load resistors on V1 and V2 in both the metal film and carbon comp varieties. Remembered I went into an electronics store to buy Allen Bradley 100K carbon comps about 20 years or so ago and they were out of 100Ks. Thought I better buy a hundred each of the 91K and 110Ks just so I had something in the 100K range. These resistors cost me a penny a piece. Later bought quite a few 100K resistors so these weren't needed until I decide to experiment with the 110K value.


Thanks
Mike

[DuaneOh]

Are you saying by lowering the grid stoppers to 10K it will significantly reduce the load on the guitar pickup.

No, It significantly increases the load if you use the lo jack input. Load was probably not a good word in that a high load (more current demand) is like a low input impedance (low resistance). In this case when plugging into the lo jack on a bassman the input impedance would be about 136k ohms, if using 10k grid stoppers instead of using the standard 68k the input impedance would be about 20k, so there would be more current demand from the pickup, a higher load.

One reason for using grid stoppers is to create a low pass filter with the miller capacitance of the tube. On the input of the amp, the low pass filter, filters out frequency's higher than the audible range, greater than 20k Hz. The 68k grid stopper and miller capacitance of a 12ax7 (about 100pf) creates the low pass filter. If you reduce the grid stopper to have you need to increase the capacitance to keep the low pass cutoff at 20k Hz, so the effect on the highs would be the same. If you have Merlin's book look at page 87.

You should experiment with different resistor types, as well as capacitors, tubes, etc. I've found that recording the amp then changing the components and recording again is the best way to compare, if you can do it blind it is even better.

[Auke Jolman]

In the last amps I've build I used a 10K grid resistor together with a 100 pF cap. I use the shield of the wire to ground the cap, which is grounded near the input jack. For me it works well and it reduces the hiss alot!

[PRR]

> If you reduce the grid stopper to have you need to increase the capacitance to keep the low pass cutoff at 20k Hz, so the effect on the highs would be the same.

The HI network is 34k resistance (two 68k parallel). Grid may be 110pFd. This comes to 44kHz. Far above the audio band; more to the point, far below the 535kHz start of the AM broadcast band (so you get >20dB attenuation of AM transmitters).

Just changing to 10k is not a big difference in hiss from 34k. It does put the hi-cut up at 120kHz, significantly less rejection of AM radio.

[thetragichero]

1w metal film are relatively cheap so just use that for the input grid and grid stopper (and maybe first stage plate load) resistors. maybe it's not very creative but for some stuff i just go with what has been proven to work instead of trying to recreate the wheel

[Mike_J]

> If you reduce the grid stopper to have you need to increase the capacitance to keep the low pass cutoff at 20k Hz, so the effect on the highs would be the same.

The HI network is 34k resistance (two 68k parallel). Grid may be 110pFd. This comes to 44kHz. Far above the audio band; more to the point, far below the 535kHz start of the AM broadcast band (so you get >20dB attenuation of AM transmitters).

Just changing to 10k is not a big difference in hiss from 34k. It does put the hi-cut up at 120kHz, significantly less rejection of AM radio.

Thank you PRR. This is what I was looking for. Hopefully with the two watt metal films there won't be much hiss anyway so it won't be an issue.

[Mike_J]

1w metal film are relatively cheap so just use that for the input grid and grid stopper (and maybe first stage plate load) resistors. maybe it's not very creative but for some stuff i just go with what has been proven to work instead of trying to recreate the wheel

Have two watt MF almost everywhere. Will test amp and change the V1 and V2 plate load resistors to 110K CCs just as an experiment. Have both types of resistors so replacing them is not a big problem. The V1 and V2 plate resistors are the only place I would consider the CCs though.

[Mike_J]

In the last amps I've build I used a 10K grid resistor together with a 100 pF cap. I use the shield of the wire to ground the cap, which is grounded near the input jack. For me it works well and it reduces the hiss alot!

That answered exactly the question I had. May give it a try if there is much hiss to remove. So to be clear you are saying you attached the shield at the ground point near the input jack to both act as a shield for the grid wire and a return for the 100pF cap from either or both of pins 2 and 7?


Thanks
Mike