Just thought I'd chime in and say that the reason the trimmer was pulled out on 124 is because we had never seen one so large in an ODS and it looked like it might have been tampered with, so we wanted to know how much it really measured.
G.
Charlie Wilson wrote:Structo, the 152k was measured from wiper to ground with the amp on and in OD mode. I just don't think you can get an accurate measurement of the overall resistance of the trimmer while it is in the circuit. I think that is why with #124 the trimmer was removed and measured. If that is the case, then even the measurement from wiper to ground with the amp on and in OD mode is not an accurate measurement of the actual resistance. It is different than the measurement with the amp off but not accurate. I hope this makes sense. I also notice that the 100k level and drive pots measure lower in the circuit than the actual resistance.
CW
In my opinion it can be, but one needs to know what one is looking at. For example, if the amp grounds the input of the overdrive when in clean mode -- most older D amps did NOT do this -- then what you'd have measuring the trimmer while in clean mode would be a voltage divider with the OD input resistor and the OD trimmer -- one can ignore the grid stopper resistor going into V1B's grid, since that's very high impedance. As long as you account for that, you can make a valid measurement for the trimmer and its setting. Now, if the amp does not ground the input of the overdrive and just leaves that network as an open circuit, like most old D amps did (*), there will be no voltage divider in clean mode and the trimmer and its setting can be readily measured -- once again, ignoring the V1B grid input impedance.
(*) In your schematic of 94, I see you've shown the OD input as grounded when in clean mode. Because of the amp's vintage and looking at the pictures of the channel switching relay, I would suspect that is not the case, however.
Cheers,
Gil
Charlie Wilson wrote:Gil,
Would that confirm my suspicion that the trimmer cannot be accurately measured for overall value while in the circuit?
CW
Thanks Gil,
Yeah all of that makes sense. I did measure the overall value and wiper to ground values for #94 while the amp was on in OD mode. So I guess those measurements are accurate. I just seems to me that I measured the same thing in my Low Plate Classic with the amp on in OD mode and the trimmer measured lower than I know it is. As far as the #94 schematic goes, I did not do that one and I think there are some assumptions made. I may get a chance to have a peek at #94 again and I will check the relay wiring.
CW
All you have to do is look at a picture of the S/N 94 relay and you will see that there aren't enough eyelets to connect the input of the OD to ground when in clean mode -- in addition to the other switching that is required. Compare that picture to the picture of any HRM amp, where the input to the OD was grounded when in clean mode, and you will see what I mean.
G.
Charlie Wilson wrote:Thanks Gil,
Yeah all of that makes sense. I did measure the overall value and wiper to ground values for #94 while the amp was on in OD mode. So I guess those measurements are accurate. I just seems to me that I measured the same thing in my Low Plate Classic with the amp on in OD mode and the trimmer measured lower than I know it is. As far as the #94 schematic goes, I did not do that one and I think there are some assumptions made. I may get a chance to have a peek at #94 again and I will check the relay wiring.
CW
Structo wrote:I thought they all grounded the OD input grid when OD is not on?
Gil is correct.. (as usual):D If memory serves Tom it was also the 2nd generation (around #40) style amps that didn't ground the OD input on clean channel as well..BTW..The relay he used at the time was the Calrad 12V DC ..I Think it was marcos who did the change over to gnd the input when on clean and reported a noticeable improvement on the clean side of his amp..So IMHO (and I believe him!) this would be a nice little upgrade for 94..Perhaps he can comment further?..
All The Best!!
Tony
" The psychics on my bench is the same as Dumble'"
There are still assumptions made about the rectifier, power supply and voltages. It also doesn't have the FET, relay power supply, relay pedal / manual switches, etc... I basically started from one of the "editable schematics" in the files section and made the modifications.
If anybody has any other corrections please let me know and I'll fix them ASAP.
I believe there'd need to be at least 7 eyelets on the OD relay board to ground the input:
1. Relay power (from relay PS)
2. Relay ground (to man/pedal switch)
3. Clean signal (from V1b coupler)
4. OD input (to 220k OD input)
5. OD output (from OD level)
6. Signal ground
7. Master vol
talbany wrote:..I Think it was marcos who did the change over to gnd the input when on clean and reported a noticeable improvement on the clean side of his amp..So IMHO (and I believe him!) this would be a nice little upgrade for 94..Perhaps he can comment further?..
All The Best!!
Tony
Found the thread where Marcos discusses how grounding the OD improved the clean sound:
Aaron you are correct. I forgot about the relay ground. I guess there is no point in a relay if you don't have a way of switching it. I like your schematic. As soon as I find my pad that has the voltages I will get them to you. They are higher than what is on the schematic. I think I also wrote down cathode voltages. The preamp cap can is missing the label so the filter caps for the preamp are going to have to be a guess although that doesn't change anything on your schematic. As far as Tony's post, I will let a braver man than me do a relay modification on #94.
CW
559]
I like your schematic. As soon as I find my pad that has the voltages I will get them to you. They are higher than what is on the schematic. I think I also wrote down cathode voltages. The preamp cap can is missing the label so the filter caps for the preamp are going to have to be a guess although that doesn't change anything on your schematic. As far as Tony's post, I will let a braver man than me do a relay modification on #94.