OD volume level in Non HRM SOLVED!

[bluesfendermanblues]

I played a gig last night using my 50w #124 clone.

The clean 'channel' was great, so was the sound of the OD.

However, the OD level was too low. A solution could be to modify the OD master volume into a HRM configuration, making it independant of the mastervolume control.

The subject was briefly discussed in the recent post here:
https://tubeamparchive.com/viewtopic.ph ... hrm#129618

Obviously, I don't want to change to sound of the OD only to have a 'real' OD master volume.

I would appreciate input to how to wire things. Have anybody done this mod? Please chip in.

[LPSGME]

It seems odd to me that the OD would not be at least as loud as the clean signal, being it is nothing more than the clean signal with even more gain added.

In any case, unless the OD tone you like depends on the OD Ratio pot being at a less than maximum level (in which case you should leave it and just add a 1meg pot for the OD's master volume) - I would think you can just replace the 100K ratio pot (as per the non HRM) with a 100k resistor and then run that maximum OD signal to a 1meg pot for the OD Master - and feed that to the OD relay for toggling between it and the clean master signal.

[img:726:526]http://i275.photobucket.com/albums/jj30 ... Master.jpg[/img]

[img:657:479]http://i275.photobucket.com/albums/jj30 ... Master.jpg[/img]

[bluesfendermanblues]

.............replace the 100K ratio pot (as per the non HRM) with a 100k resistor and then run that maximum OD signal to a 1meg pot for the OD Master - and feed that to the OD relay for toggling between it and the clean master signal.

HI LPSGME
Ok, that solves conversation of the OD output section, but how about the input section??


a) The HRM input is (470k+100k+250k+4.7k=) 824.7K parallel to the Clean master pot of 1m, giving a total input impedance of 452k

b) Whereas the ODS has an input impedance of (220k + 100k =) 320K
Should the ODS 220k be upped to 352k (which paralled to the 1m pot will be 320k) in order to get the same load of V1b.

[talbany]

In a low-level signal application, you want maximum voltage transfer,(not necessarily gain)


The "ideal" situation is a zero output impedance from the driving source going into an infinite impedance for the receiving end.In general, if the source impedance is at least 10 times lower than the load impedance,(pot's value) you're okay, because you'll get 90% of signal transfer.(This can still work in the opposite direction and dump voltage)

Impedance also effects frequency response The lower the source impedance, the less high end you'll lose due to load capacitance on the input of the following gain stage or tone stack pot etc.. I digress..

Crude example
In the case of the V2A OD gain stage of a Non HRM a 12AX7 has a plate impedance of 80k add the plate resistor of a low plate 100k and the 100k of the series resistor before the ratio or drive pot you have a source impedance of at least 280k run that into a 100k pot your in effect cutting or dumping almost 1/3 voltage even with the pot set all the way up..

This is how I understand it..

Hope this helps

[greiswig]

I played a gig last night using my 50w #124 clone.

The clean 'channel' was great, so was the sound of the OD.

However, the OD level was too low.

How high are you setting your clean volume? In my low plate non-HRM, I end up with it about 1:00-2:00. If I get much higher than that trying to get the clean side to break up a little, its volume quickly swamps the OD channel.

[bluesfendermanblues]

I played a gig last night using my 50w #124 clone.

The clean 'channel' was great, so was the sound of the OD.

However, the OD level was too low.

How high are you setting your clean volume? In my low plate non-HRM, I end up with it about 1:00-2:00. If I get much higher than that trying to get the clean side to break up a little, its volume quickly swamps the OD channel.

I play a strat (with a SD 'twangbanger' in bridge pos - great for a tele like lead tone, by the way).

The Clean Volume is around 7 (1:30)
Tone controls around noon
Input trim around 20k
OD gain on 4 (10:00)
OD Master crancked

[Structo]

That may be one of the reasons that HAD switched to the HRM with the separate masters.

[bluesfendermanblues]

That may be one of the reasons that HAD switched to the HRM with the separate masters.

Agree, especially if you like to run the input vol up pretty high...

[LPSGME]

.............replace the 100K ratio pot (as per the non HRM) with a 100k resistor and then run that maximum OD signal to a 1meg pot for the OD Master - and feed that to the OD relay for toggling between it and the clean master signal.

HI LPSGME
Ok, that solves conversation of the OD output section, but how about the input section??


a) The HRM input is (470k+100k+250k+4.7k=) 824.7K parallel to the Clean master pot of 1m, giving a total input impedance of 452k

b) Whereas the ODS has an input impedance of (220k + 100k =) 320K
Should the ODS 220k be upped to 352k (which paralled to the 1m pot will be 320k) in order to get the same load of V1b.

I just tried it and the slight difference I detected by hanging a 1 meg pot off the .05 cap at V1b's output seemed to be restored by increasing the resistance going to the OD input.

[Bob-I]

Crude example
In the case of the V2A OD gain stage of a Non HRM a 12AX7 has a plate impedance of 80k add the plate resistor of a low plate 100k and the 100k of the series resistor before the ratio or drive pot you have a source impedance of at least 280k run that into a 100k pot your in effect cutting or dumping almost 1/3 voltage even with the pot set all the way up..

This is how I understand it..

Hope this helps

Me too. And in that example a simple change to a 250K pot as a drive pot may solve the problem. Worked in mine, now I have great control on both channels.

[bluesfendermanblues]

Crude example
In the case of the V2A OD gain stage of a Non HRM a 12AX7 has a plate impedance of 80k add the plate resistor of a low plate 100k and the 100k of the series resistor before the ratio or drive pot you have a source impedance of at least 280k run that into a 100k pot your in effect cutting or dumping almost 1/3 voltage even with the pot set all the way up..

This is how I understand it..

Hope this helps

Me too. And in that example a simple change to a 250K pot as a drive pot may solve the problem. Worked in mine, now I have great control on both channels.

Thanks for your input, Tony 'Talbany' and Bob-I, I'll try swopping the 100k volume pot for a 250k and tell you how it goes.

[ayan]

I played a gig last night using my 50w #124 clone.

The clean 'channel' was great, so was the sound of the OD.

However, the OD level was too low. A solution could be to modify the OD master volume into a HRM configuration, making it independant of the mastervolume control.

The subject was briefly discussed in the recent post here:
https://tubeamparchive.com/viewtopic.ph ... hrm#129618

Obviously, I don't want to change to sound of the OD only to have a 'real' OD master volume.

I would appreciate input to how to wire things. Have anybody done this mod? Please chip in.

What you describe can happen. I suggest you replace the output of V2B after the .005uF capacitor. In a stock amp, it is a 150K fixed resistor (sometimes 180K) followed by a 100K pot. Short the resistor out and replce the pot with a 250KL pot and you will keep the same sound, while not getting the feeling that your OD volume control has to be all the way up (which would kill me psychologically, if nothing else) to match the clean channel's output. Also, you will have a lot more volume available on tap.

Gil

[bluesfendermanblues]

I played a gig last night using my 50w #124 clone.

The clean 'channel' was great, so was the sound of the OD.

However, the OD level was too low. A solution could be to modify the OD master volume into a HRM configuration, making it independant of the mastervolume control.

The subject was briefly discussed in the recent post here:
https://tubeamparchive.com/viewtopic.ph ... hrm#129618

Obviously, I don't want to change to sound of the OD only to have a 'real' OD master volume.

I would appreciate input to how to wire things. Have anybody done this mod? Please chip in.

What you describe can happen. I suggest you replace the output of V2B after the .005uF capacitor. In a stock amp, it is a 150K fixed resistor (sometimes 180K) followed by a 100K pot. Short the resistor out and replce the pot with a 250KL pot and you will keep the same sound, while not getting the feeling that your OD volume control has to be all the way up (which would kill me psychologically, if nothing else) to match the clean channel's output. Also, you will have a lot more volume available on tap.

Gil

Just checked the value of the fixed resistor. It's a 150k, and the pot is 100k, so I'll change the pot to 250 and loose the resistor. I'll keep the sound - Great ! Looking forward to having a little reserve on the OD level.

[Structo]

I'm pouring over the schematics I have on hand and from what I can tell, the correct way for an HRM amp to be wired is to have the Clean Master before the Clean/OD relay.
So that when it switches to OD, the Clean Master volume pot is taken completely out of circuit.
So that the two clean stages feed directly into the OD stages.
Then the OD volume or OD Master Volume controls the volume.
It is still dependent on the Pre-Gain volume for overall volume but this way the Clean Master and OD Master are independent of each other.

I notice quite a few of the HRM schematics I have do not show this correctly.

Claus this may be an alternative to your problem and would only require rewiring where the Master Volume resides in the circuit.

The schematic I attached shows this the clearest of the many I have.

[LPSGME]

Tom,

Doesn't that circuit still leave the 1 Meg clean master hanging on the OD input?

The only way I can think to wire the relay, so that it doesn't, would leave off the the ability to ground the OD input - like this.

[IMG:500:500]http://i275.photobucket.com/albums/jj30 ... lean-1.jpg[/img]

Otherwise, regarding all the talk of impedance etc. I don't think there is any one 'right' set of resistor or pot values. Changing them slightly imparts different a tone and response that can be used in best match to the one's guitar or and the pickups etc.

I'm also using 250K (rather than 100k) on the Drive or Level pot, but I've opted to use different input and output resistors as well.