OD HF Taper rolls highs off of clean?

[Charlie Wilson]

Hello, I was playing around with an OD high frequency taper in my low plate classic and noticed that it rolls high end off of the clean channel also. I have read about this in other posts, I hear it in my amp but I don't see how it is possible. Anybody else notice the same thing and any explanations?
CW

[bluesfendermanblues]

I haven't noticed that on my own amps - since the HF trimmer is internal, I need to take the chassis out of the cabinet in order to adjust the trimmer and make some testing.


Hypothsis I: You could have connected the OD relay backwards, grounding the output in stead of the input.

Hypothesis II: The only connection the OD has to the clean circuit, when not engaged, is through the Powersupply - through the V2b anode resistor.

Meaning that the HF trimmer would impact the circuit through the anode resistor. This seems unlikely, since the HF trimmer would only add a little extra capacitance to the total capacitance of the PSU caps.


My apriori expectation would be Hypo I.

[Charlie Wilson]

Thanks BlueFender. I am curious about possibility #1. Even if the OD input was not grounded isn't it still isolated from the clean side?
CW

[marcos]

Hi Charlie,
I have also noticed this phenomenon. After discussing this on the forum,
I changed the wiring on my personal amp (1979 ODS) to the 80s style wiring where the OD input is grounded. I still hear a difference when connecting
either a "h.f. taper" circuit, as in some Dumbles, or larger value snubbers,
although the difference in tone is very subtle. I have no explanation for this,
might well be the connection via the power supply.

Marcos

[Charlie Wilson]

Thanks Marcos. That is interesting. Yeah the effect is less on the clean channel. It actually works to my advantage as my clean side is a bit bright and a slight high roll off on the clean and more of a roll off on the OD sounds good. I am only using a 100pf or so cap from the OD volume to ground.
CW

[ic-racer]

Can you post a schematic and or layout or picture?

[Charlie Wilson]

Hello Ic-racer. I am not sure whether you mean for the amp or just the little HF taper. I attached a layout for the Low Plate Classic. All I have going is a 100pf cap from the ratio pot(mine is volume) terminal B to ground. It is just clipped in for now. Even with that small a cap I can hear a difference on the clean side. I am certain that my OD relay is wired correctly and Marcos confirms that others have experienced this. It must be through the power supply because the OD is isolated everywhere else when in clean mode. The thing is, if that is the case then the two OD caps (.01 and .005) are also interacting with the clean side since they have a path to ground across the volume and drive pots. I wonder what the clean side would sound like with the two OD plate resistors lifted?
CW

[ic-racer]

I wondered if it was the same place as in #094 and I guess it is. Odd behavior, it does not seem to be connected in any way to the clean circuit.

[jam-mill]

If the HF taper circuit is affecting the cleans then wouldn't rotating the ratio pot have a similar effect?

According to this schematic (https://tubeamparchive.com/download/file.php?id=16859), both the input and the output of the OD circuit are electrically isolated via the OD relay.

How can the HF taper capacitance be coupled through the power supply? Aren't CL and OD triodes on different B+ nodes?

[Charlie Wilson]

Yeah Jack it is odd. On paper it doesn't make sense to me but I hear it. If you ever have your amp open take a good sized cap and clip it from the OD volume to ground and listen to your clean channel. If you don't hear a difference let me know because then I may have wired something wrong in my amp.

CW

[jam-mill]

Yeah Jack it is odd. On paper it doesn't make sense to me but I hear it. If you ever have your amp open take a good sized cap and clip it from the OD volume to ground and listen to your clean channel. If you don't hear a difference let me know because then I may have wired something wrong in my amp.

CW

Hi Charlie,

I made a video showing the o'scope trace while running a 1KHz signal into my amp while rotating the HF Taper (I have it on the front panel) and switching on/off the OD. It clearly shows no change in the signal level when the OD is bypassed. Let me know how to send it to you.

Also, if I am not scheduled to play in church this weekend, was planning to open up the amp again to install a relay for controlling the mid boost and can also do the large cap test you requested.

EDIT: uploaded the movie here

-jack