# 50-500pf 50v cap on PI plates



## Mikev7305 (Jan 6, 2020)

I'm trying to add a small value cap in parallel with my phase inverter plate load resistors on my yba-2 and I ordered a few of these: 









Capacitor - Ceramic Disc 50V (choose value)


Stable, Low Cost Ceramic Capacitor Accuracy: ±20% Wide Operating Temperature Range - +10oC to +85oC Dielectric Withstand - 2.5x Rated Voltage Body Diameter - 4mm Pin Spacing - 3mm




nextgenguitars.ca





But I'm not sure that a cap that's only rated 50v is good enough there. They don't have any other capacitors with values of 50pf-500pf. 

Will these work?


----------



## Latole (Aug 11, 2020)

YBA-2 shematic 



https://el34world.com/charts/Schematics/files/Traynor/Traynor_bassmate_yba2.pdf


----------



## Paul Running (Apr 12, 2020)

If the phase inverter out swing, is less than 50VP-P, the cap should handle it. Why not scope those points to observe the signal...look for high-frequency signals...check it without the cap. You may not need it in circuit. Remember every component added, affects the tone.


----------



## Latole (Aug 11, 2020)

If op ask about cap voltage value, he may not have a scope IMO


----------



## Paul Running (Apr 12, 2020)

Latole said:


> If op ask about cap voltage value, he may not have a scope IMO


Yeah, I keep forgetting about that. I remember it took me a long time to get my first scope however nowadays, they are affordable, once you use a scope, you realize what you have been missing.


----------



## Mikev7305 (Jan 6, 2020)

Correct I do not have a scope lol. I'm very new to this. I'm just trying to cut a bit of harsh highs when it gets turned up. 

I'll try the cap there tonight. Thanks guys!


----------



## Latole (Aug 11, 2020)

" I'm just trying to cut a bit of harsh highs when it gets turned up." 

And PI circuit is the right circuit to work on ?


----------



## Mikev7305 (Jan 6, 2020)

@Latole isn't that cap (25 25 - c3) a cathode bypass cap? There would be pretty different voltage going on there right? Im always hesitant to poke around on the live amp to check voltages.


----------



## Mikev7305 (Jan 6, 2020)

I'm trying to do the tweak as late as possible in the circuit, but im not confident enough to work in the power section yet. So phase inverter it is


----------



## Latole (Aug 11, 2020)

Mikev7305 said:


> @Latole isn't that cap (25 25 - c3) a cathode bypass cap? There would be pretty different voltage going on there right? Im always hesitant to poke around on the live amp to check voltages.



My bad, sorry, you write about plate, not cathode........
You need at least a 400 volts caps. Those ceramic are available to 1K volts.

Are you sure all tubes bypass caps are good ?


----------



## Latole (Aug 11, 2020)

Harsh highs ; too cold bias on 6V6 could also be the ( part of ) issue ?


----------



## nonreverb (Sep 19, 2006)

Here's the first thing you should do if you feel you really need it. Measure the differential DC voltage between the two plates. I've never seen the two voltages exactly the same. If they're close, no problem. If it's out by 30 or 40VDC, I'd either replace one of the 100K plate resistors to get them closer or use a 100V rated cap.


----------



## nonreverb (Sep 19, 2006)

Like Paul Running mentions, the P-P AC must also be considered. Therefor the AC P-P plus the DC differential will dictate the voltage spec of the cap.


----------



## Mikev7305 (Jan 6, 2020)

I'll measure the plate voltages tonight 😬 after a youtube video or two of course. 

I'm so grateful for the advice you guys have to offer, but every time I ask a question I realize further that I should just stick to playing the damn thing lol. Slowly I am learning though. Thanks again guys!


----------



## Markus 1 (Feb 1, 2019)

The standard practice is high voltage picofarad caps on the plate resistors.
Get 400 to 500 volt rating
Check Justradios.com
Canadian outfit in Scarborough


----------



## loudtubeamps (Feb 2, 2012)

What you are looking for is the last place for a tweak? ...this would be it.
If the schematic provided by *Latole *is the same circuit as yours...you may want to try a negative feedback resistor between the phase inverter and secondary of the output transformer .
You could use a 500k-1 meg pot and sweep to find a sweet spot ,if any.
If you try this, lift one side of the 100pf cap as well ..find the sweet spot with the neg. feedback then drop the other leg of the 100pf cap back into the circuit, and have a listen to the difference, you may end up not needing the cap at all.
The guys here with more experience would be able to instruct you as to the best place to connect to the phase inverter .
An example here.... https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-5D4-Schematic.pdf
Something else to consider....what kind of speaker are you using?


----------



## Latole (Aug 11, 2020)

Did you fin your amp is little noisy too ? Like a hum or hiss ? 

Transformer heater winding must have a center tap to ground ; you can build one with two 1/2 watts 100 ohms or 220 ohms resistors.




__





The Valve Wizard






www.valvewizard.co.uk














For your safety and less noise, add a 3 prong grounded power cord

Replace boths input jack with one with normaly close switch to ground as schematic by loudtubeamps
We call them shorting input jack


----------



## Latole (Aug 11, 2020)

Mikev7305 said:


> I'll measure the plate voltages tonight 😬 after a youtube video or two of course.
> 
> I'm so grateful for the advice you guys have to offer, but every time I ask a question I realize further that I should just stick to playing the damn thing lol. Slowly I am learning though. Thanks again guys!



*Be very careful,* lethal voltage on tubes's plate


----------



## Mikev7305 (Jan 6, 2020)

loudtubeamps said:


> Something else to consider....what kind of speaker are you using?


I've tried it through a g12t-75, eminence gb128, and a warehouse et65, the harshness is there in all of them through certain guitars. 



Latole said:


> Did you fin your amp is little noisy too ? Like a hum or hiss ?


The amp isn't noisy. It has had work done on it fairly recently. There's a 3 prong cord and updated electrolytics. Among a couple other odd things... 
My first plan was to install a cut control, until I decided to try the picofarad caps on the PI plates, but since my caps won't work, I might hook up a cut control temporarily and see if it does the trick.


----------



## GreasyB in YYC (Jan 6, 2020)

Mikev7305 said:


> Correct I do not have a scope lol. I'm very new to this. I'm just trying to cut a bit of harsh highs when it gets turned up.
> 
> I'll try the cap there tonight. Thanks guys!


Might be worth jumpering in a conjunctive filter (resistor+1000v cap between the power tube plates) to see if you like it. It is a late-in-the-circuit high frequency attenuator and eliminates some hiss. The Secret of Dr Z's Amp Mojo - RCA Conjunctive Filtering Design Guide


----------



## Mikev7305 (Jan 6, 2020)

GreasyB in YYC said:


> Might be worth jumpering in a conjunctive filter (resistor+1000v cap between the power tube plates) to see if you like it. It is a late-in-the-circuit high frequency attenuator and eliminates some hiss. The Secret of Dr Z's Amp Mojo - RCA Conjunctive Filtering Design Guide


That was a great read. And something Ive noticed about my buddy's maz 18. But for now all I wanted to do was cut this sharp fizzy ultra high frequency when it's cranked. I just finished installing a cut control on it. I couldn't crank it, but the low volume test seems promising. 

If it doesn't work, I'll simply turn that pot into a master volume as I wanted to try that anyways. 

I'm going to look up more of that conjunctive filtering and see if it's something I should (could) do.


----------



## loudtubeamps (Feb 2, 2012)

Another quick trick: disconnect cathode bypass caps.
One at a time..... lift C1 from ground and have a listen if C1 doesn't knock down the fizz, reconnect and try C3,it might be all you need.
And remember re:the neg. feedback resistor, a very easy test and think of how a presence control affects the top end.


----------



## nonreverb (Sep 19, 2006)

GreasyB in YYC said:


> Might be worth jumpering in a conjunctive filter (resistor+1000v cap between the power tube plates) to see if you like it. It is a late-in-the-circuit high frequency attenuator and eliminates some hiss. The Secret of Dr Z's Amp Mojo - RCA Conjunctive Filtering Design Guide


I thought this sounded familiar! They used conjunctive filtering on some Hammond organ power amps.


----------



## Paul Running (Apr 12, 2020)

The first Dr. Z Carmen Ghias were made using Hammond organ reverb amps.


----------



## Paul Running (Apr 12, 2020)

Semi-conductor rectifiers produce HV ringing, especially the types with very fast, slew rates,


----------



## nonreverb (Sep 19, 2006)

Paul Running said:


> The first Dr. Z Carmen Ghias were made using Hammond organ reverb amps.


The Hammond AO-35. I've done a Carmen Ghia mod on one and am doing another for a customer soon. Very cool amp.


----------



## Paul Running (Apr 12, 2020)

A corrective filter is used in open-loop designs, when negative-feedback is not used.
It is used to correct the frequency characteristics of an output stage that uses screen-grid tubes. The filter consists of a resistor and a capacitor, connected in series across the primary of the output transformer. The filter is in parallel with the plate-load impedance which is reflected through the transformer by the voice-coil (transformer-action). The value of reflected impedance *increases* with frequency in the middle to upper audio range.
Inversely, the impedance of the filter, *decreases* with increasing frequency. In theory, the center-values for the resistance and capacitance in the filter and the load impedance on the output tubes can be regulated for middle to upper audio frequencies.
The resistor value is 1.3 × plate-to-plate load resistance for push-pull or 1.3 × plate load resistance…check the data sheet!
The capacitance in the filter should have a value such that the frequency response is flat from 400 to 1KHz in the output stage. That may change according to the application of the amp.
A practical method to determine the value of capacitance for the filter is: take 2 measurements of the output-voltage across the primary of the output transformer: first, with a 400Hz signal and then with a 1KHz signal, double-check that the input level signal is equal for both measurements…I would use a scope. The ideal capacitor is one that provides equal output voltage for 400 and 1KHz, in this case…much like a band-pass filter.


----------



## Paul Running (Apr 12, 2020)

Ghia, said to be inspired by those Hammond Reverb amps


----------



## jb welder (Sep 14, 2010)

GreasyB in YYC said:


> Might be worth jumpering in a conjunctive filter (resistor+1000v cap between the power tube plates) to see if you like it. It is a late-in-the-circuit high frequency attenuator and eliminates some hiss. The Secret of Dr Z's Amp Mojo - RCA Conjunctive Filtering Design Guide





nonreverb said:


> I thought this sounded familiar! They used conjunctive filtering on some Hammond organ power amps.


Gar Gillies used the conjunctive filter in several Garnet circuits as well. Pro and BTO for example.


----------



## nonreverb (Sep 19, 2006)

jb welder said:


> Gar Gillies used the conjunctive filter in several Garnet circuits as well. Pro and BTO for example.


Just looked over the schematics to reacquaint myself....Conjunctive filter and NFB too!


----------



## nonreverb (Sep 19, 2006)

Interesting...I wonder why he chose ad them to those particular amps?


----------



## GreasyB in YYC (Jan 6, 2020)

jb welder said:


> Gar Gillies used the conjunctive filter in several Garnet circuits as well. Pro and BTO for example.


And a modified version (cap only) in the 60’s Herzog.


----------



## Mikev7305 (Jan 6, 2020)

So I considered the idea of conjunctive filters, so much so that I bought myself a maz 18 😎. It was a bit more expensive than the cost of a couple capacitors and resistors... But why not right?! I won't get my hands on it until Christmas though. NAD thread to be expected through the holidays


----------

