Hot Cat 30 out of higher voltage EL34 amp

[bcmatt]

I've been obsessing lately over the Hot Cat 30. I'd sort of like to try and modify one of my old Traynors into one if possible.

I'm wondering what my biggest issues might be. I'm mostly concerned about trying to cathode bias some EL34s that perhaps this PT wants to give much higher plate voltage. Do you guys think this PT could ever make some form of Hot Cat circuit, with diode rectifier and Resistors instead of a choke?

I'd also like to figure out a way to use potentially a couple VVR circuits to control the Power Tube Plate voltages.

Here is the current power supply (it currently has VVR for the Screens to run a pair of KT88s):

YGL with KT88 screen VVR.jpg

Here is the Hot Cat Schematic:

badcat_hotcat-30_sch.pdf

Thoughts?
I'm mulling over whether there is some way I could spread out heat by using a couple VVR circuits. One internal that knocks down the plate voltage a bit to get to the standard Hot Cat voltages, then another one controlled on the front panel to use just for controlling the power tube plates.... I'm not sure.
I guess my first question is more about whether this amp could become some sort of Hot Cat without the need for VVR.

[B Ingram]

I've been obsessing lately over the Hot Cat 30. I'd sort of like to try and modify one of my old Traynors into one if possible. ...

The Traynor PT seems to deliver 60vdc more than you need.

You Do You, but I never enjoyed projects where I had to jury-rig the power supply due to using the wrong power transformer. I would save up my Ducats until I could buy a power transformer that delivers the voltages I want.

I'd also shoot lower than I think I need on PT voltage. I never saw any forum complaint about "My B+ is too low" but I've seen many-hundreds trying to figure out how to waste off some of a too-high B+ voltage.

[maxkracht]

Correct power transformer for the job is definitely best, but if you're going ahead anyway, heres another trick to knock off 24v at the power tubes. https://www.tubecad.com/2009/04/blog0163.htm

[bcmatt]

The Traynor PT seems to deliver 60vdc more than you need.

You Do You, but I never enjoyed projects where I had to jury-rig the power supply due to using the wrong power transformer. I would save up my Ducats until I could buy a power transformer that delivers the voltages I want.

I'd also shoot lower than I think I need on PT voltage. I never saw any forum complaint about "My B+ is too low" but I've seen many-hundreds trying to figure out how to waste off some of a too-high B+ voltage.

I hear you. I'm more in the situation that I don't at all need a new amp. It's more about turning the amps I have into amps I like more. I have my main amps that I use the most (Liverpool, DC30, Express) This one amp has a few mods already, and currently is running a pair of KT88s. I might just experiment a bit more with the power section first, but the plan is to try a completely new preamp eventually.
Maybe I'll see what it is like cathode biasing the KT88s first and see how things land.

[bcmatt]

Correct power transformer for the job is definitely best, but if you're going ahead anyway, here's another trick to knock off 24v at the power tubes. https://www.tubecad.com/2009/04/blog0163.htm

I was thinking I would probably do the Garter circuit again on this amp anyways. The main challenge is needing twice as many resistors while I'm still trying to dial in the right resistor value. Then on this amp, I imagine they need to be significantly higher wattage too. I forgot that it knocks off a bit more voltage. It wasn't much of a factor on the last amp since it had VVR first and I always end up running it below 50% plate voltage anyways.
This amp may never be able to run VVR, but that is the ultimate dream...

[maxkracht]

Forgot I was talking to you about the garter thing before, my memory isn't the best... Yeah, lot of space for 10w resistors. You might consider some 10-15w chassis mount resistors. They are physically smaller than normal 10w wirewounds and chassis mount might give some more options in 3d space. I generally tap them for M3 screws, not a lot of room for a nut on those. You can also ballpark the resistor value then add another in series to fine tune things. The added resistor doesn't need to be very high wattage if it's only a few tens of ohms, but yes, still pretty annoying to add/match another 4 resistors.

[bcmatt]

I don't think I am nearly as worried about space with this amp. The chassis is quite huge, and I plan to pull out two of the power tube sockets. If I manage to get this working ok with cathode bias, I guess I would be making use of the space to try and thoroughly heat-sink a VVR circuit. I'm not stuck on using the KT88s at all, but I am curious about how they would sound cathode-biased vs the EL34s. But realistically, if I'm trying to get VVR in this circuit for the power tube plates, I may want to give up the space that the current VVR for the Screens is using.

With 470V on the plates of the 2x KT88s
the VVR is helping give 300V on the Screens . That is 2 x 240 ohm 10W power resisters in Series before the VVR circuit, and 22 ohm screen resisters on the two sockets I'm using.
They are dissipating about 40mA (38 and 40), so it's set to about 54% of 35W KT88s.

I would imagine that cathode biasing up around 90% dissipation would pull the plate voltage down some. I'm not sure how much.
I also should install some thermistors to get the input voltage down a few volts to the amp. I need to measure, but I suspect this amp from the 70s is riding heater voltage rather high with modern wall voltages. I should get this thing open and take some voltage readings. I'll also check what I currently have for power resistors.

[bcmatt]

So, while looking at this amp and its inability to add a tube rectifier and it's necessity to use a bridge rectifier because of the PT's lack of HV centre-tap, I've started to adapt the Hot Cat Schematic to what this might look like, including my desire to use the VVR with a massive heat-sink.

I'm still trying to figure out what I might use for filtering capacitors and B+ node resistors since I think I'd also like to avoid buying a choke. I was looking a little bit at the Express schematic for some inspiration as well.

Hot Cat Adapted from YGL Questions.png

I'm thinking of 4 questions right now though:

1) If using a bridge rectifier with the artificial centre tap, is it better to keep the Standby switch where the Traynor currently has it where it lifts the ground from that Bridge Rectifier? Or have it after the rectifier and first Filter caps?

2) I see the Hot Cat sends B+ to Screens and Plates of the EL34s from the same B+ node with just 10 Watt 1K Screen Resistors only. Something like the Express does what I feel is more normal with a 25W 1K resistor before sending to the screens with additional 5Watt 1K Resistors on Each Screen. What would be the advantages of each? Is the Express Significantly safer in that department? According to the schematics, the Express drops 15Volts from B+1 to B+2 before it even sends to the Screen Resistors. The Hot Cat Schematic suggests its actual screens end up about 9 Volts less than its plates.

3) What should I keep in mind when replacing the choke with a Resistor? Just try something like 1K? I imagine it depends on whether I copy the first few nodes of the Express more closely, separating the Plate and Screen sends...

4) Any thoughts on my Sag Resistor approach? I realize that also affects everything else and it will all require some experimenting, but just trying to come up with an initial plan.

I would probably play with the power section experiments to get the filtering and cathode biasing right before gutting the preamp to build into the Hot Cat.

[bcmatt]

So, even just looking at some of the parts I have on hand already, this is what I'm thinking of changing. (Also the B+ node order and labelling was driving me a bit nuts, so I had to change that)

Hot Cat Adapted from YGL Filters.png

I don't think I want to order a choke if I can avoid it, since that alone would likely cost about $50 plus shipping. So I'm thinking of adapting this a little to look a little like the Chieftan, and more like the Express when it comes to filtering. I'm not sure if it is enough, or too much, but I imagine it might not be too far off.
Once I think I have the right Cathode Resistor values, I'll also probably switch it over to the Blumlein Garter bias circuit, so I'll have to get 4 of that value.

[bcmatt]

To give this VVR Mosfet the best chance of survival, I'm thinking I will mount it directly to this heatsink through an old Tube socket hole in the chassis. Then the fan will blow through it on the way to blowing on the 2 remaining EL34s:

Heatsink.jpg

Heatsink2.jpg

[maxkracht]

Computer heatsync and vintage fan is giving some solid mad scientist vibes

[bcmatt]

Well, the heatsink fits in the head cab with half a millimetre to spare. I still have to drill the hole for the mosfet boltm, but I'm trying to decide if I need to cut a hole in the chassis so that it will mount squarely in the middle of the copper pad, or if it can straddle the aluminum and copper parts of the heatsink. I feel like cutting the steel chassis will be a PITA and filings will get everywhere. I read copper conducts heat about three times as well as aluminum, but most heatsinks are just aluminum...

20250317_184540.jpg

20250317_184652.jpg

20250317_184720.jpg

[solderhead]

I'm digging that cooling system. You're definitely thinking outside of the box.

Is there a clamp-on fan/shroud combination that's an OEM solution for that CPU cooler? I would expect that a 120mm shrouded fan that clamps onto the heatsink would give better forced-air cooling than a solution that uses a non-ducted fan.

[bcmatt]

Is there a clamp-on fan/shroud combination that's an OEM solution for that CPU cooler? I would expect that a 120mm shrouded fan that clamps onto the heatsink would give better forced-air cooling than a solution that uses a non-ducted fan.

I don't think this particular heatsink had any fan mounting. I actually think it's pretty old before they became standard to mount fans to. I've had this heatsink in a container of parts for over a decade, and at the time I got it, it had come out of some old no longer used PC.

[solderhead]

google matches that heatsink part number to a Dell W4254 heatsink assembly for the OptiPlex GX280.

the problem with Dell CPU coolers is that they often used model-specific ductwork instead of generic clamp-on fans like you might find on a generic CoolerMaster.

That said, the square holes in the top are where they often mount the custom shrouds (tabs on the shroud fit into the square holes), or where spring-type retaining clips would grab-on to hold a clamp-on fan onto the heatsink body.

I'd bet that you could fabricate some kind of clamp to hold a fan directly against the fin assembly if it turns out that you should need it.

Just a thought.