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My first amp project was in the form of "marshallizing" an epiphone valve junior, and one of the things I did at the time was put in a switchable conjunctive filter. It really smoothed out the edginess and was great for lead tones. The CF is something I think that has been popularized, to an extent, by Dr Z since he incorporates this into the CG. From my research it seems that this idea is also not something new either...it was described in a handbook way back in the 1930s.

I'm in the tweaking process on my 18-watt Superlite II and thought I would research this a little bit, and came across a "snubber" kit from AES:
https://www.tubesandmore.com/sites/default/files/associated_files/snubber_article_k-snub.pdf

It is similar to yet different from the CF that is mentioned in most other designs. From what I've read, a CF is simply a cap and resistor (of appropriate value and voltage/wattage) in series across the OT primaries. However, this snubber circuit appears a little different...there's a 5W 3K resistor coming off each plate of the output tubes, and a cap/8.2M ohm resistor in parallel for each tube, but connected in series with each other (hard to explain but the article above shows this).

Are these essentially accomplishing (or trying to accomplish) the same thing? I was considering just installing a 5W 10K resistor and a cap in series as shown in a typical CF, but was wondering if there is any advantage to the snubber circuit over this design. The more simplistic CF approach would allow for easier experimentation with cap values to achieve the desired level of "smoothing" compared to the relatively more complex snubber circuit.

I've read over some threads on this forum as well as EL34 world and 18 watt, but didn't see anything that addressed the snubber and CF and compared the pros/cons of each.

Any insight you might be able to give on the topic is greatly appreciated. Thanks!

In the snubber example the plate has the OT primaries connected to it so that "snubber" circuit is connected in the same way as the conjuctive filter mentioned.

Both are simple first order RC filters.

Note that snubber circuit from the magazine could be simplied down to a two resistors and cap.

Both circuits are doing the same thing, just different names.

If you do some more searching you'll find people call shunt capacitors snubbers too. You'll often see these shunt snubbers across plate resistors, across PI outputs, from plate to grid, from plate to cathode, etc.

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http://pdfelectronics.com

The CF is a simpler circuit that performs a similar function. They both smooth out the OT. We used one in the TriFly.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

Thanks for the info guys. I'll try out the CF and see how it turns out. I don't want to give up the edgy 18-watt tone completely so I might make it switchable if it turns out that I get something that sounds good.

For that, I would start with the simple CF as ssed on a Dr.Z c@rmen ghi@.

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I will start with a 10K 5W resistor in series with a mallory 2.2nF cap. Might go to 4.7nF and 1nF for comparison, then possibly make resistor changes if I still haven't dialed something in. From what I've read, the resistor value should be 1.3 times the impedance, so that puts the resistor close to 10K. I may adjust slightly but I have a feeling that the cap will make the biggest difference.

The RCA tube manual has information for the conjunctive filter listed as "corrective filter". It reads as follows:

"A corrective filter can be used to correct the frequency characteristic of an output stage using a beam power tube or pentode when inverse feedback is not applicable. The filter consists of a resistor and a capacitor placed in series across the primary of the output transformer. Connected in this way, the filter is in parallel with the plate load impedance reflected from the voice-coil by the output transformer. The magnitude of this reflected impedance increases with increasing frequency in the middle and upper audio range. The impedance of the filter, however, decreases with increasing frequency. It follows that by use of the proper values for the resistance and the capacitance in the filter, the effective load impedance on the output tubes can be made practically constant for all the frequencies in the middle and upper audio range.
The resistance to be used in the filter for a push-pull stage is 1.3 times the recommended plate-to-plate load resistance; or, for a single-tube stage, 1.3 times the recommended plate load resistance. The capacitance in the filter should have a value such that the voltage gain of the output stage at a frequency of 1000 cycles or higher is the same as the voltage gain at 400 cycles.
A method of determining the proper value of capacitance for the filter is to make two measurements of the output voltage across the primary of the output transformer; first, when a 400-cycle signal is applied to the input, and second, when a 1000-cycle signal of the same voltage as the 400-cycle signal is applied to the input. The correct value of capacitor is one which gives equal output voltage for the two signal inputs. In practice, this value is usually found to be on the order of 0.05uF."

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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps

I set it up with a DPDT center off switch so I was able to compare it to the non-CF, unaltered tone. The switch was wired with a 2.2nf and 10nf both going through a 5 watt 10K resistor. The 2.2 was slightly noticeable but did take some of the edgy/fizzy off, sounding good...smoother but still in your face a little. The 10nf was way too dark...almost like a blanket was over the speaker. I will probably pull the 10nf cap and replace it with a 4.7 and see how that sounds. At the very least I will keep the 2.2 switchable. Overall this turned out to be a good mod to the amp.