Hot Cat 30 out of higher voltage EL34 amp

[Colossal]

Hi Matt,

Yes, same layout as the Songwriter.

I've been doing some SPICE modeling of the SW bass control but in the context of a Hot Cat-like build, meaning, trying to create a sweep that is not too trebley or bassy and with a bit better response. Your work has confirmed that 1n5 is a comfortable spot for a single coupling cap value. The SW bass control produces a widely shelved response, rather than the classic -3dB drop of single, given coupling cap value. The SW also had the fixed treble peaker of 220p/330k which definitely bumps the treble up. The Hot Cat is more wide open but with a tight, controlled bass that starts to roll off around 300Hz. SPICE is showing that with 2n2 for the fixed capacitor and above that, in parallel, a 10n in series with 500k (maybe linear would be better) would create a similar roll off point, around 300Hz or so, like the fixed 1n5, and with the Bass knob turned up full, 7.2dB down to 50Hz, and turned all the way down, to about 3.7dB at 70Hz. Keeping the lower, fixed capacitor at a higher value (2n2 in place of the SW's 500p) keeps the gain in the midrange to low-mid frequencies. 500k does limit how much bass is rolled off, where 3M is going to give a deeper cut. When I was fiddling with a friend's SW30, his bass pot measured 3.7M but I didn't find what I was looking until after noon on the dial. The capacitance measured 8nF and above after that. About 2pm was the sweet spot (around 10nF or better). But again that amp had the treble peaker. We soldered in 2n2 on a switch over the treble peaker as a mid boost and it was better. So that tells me, I am going to prefer the more wide open nature of the Hot Cat's pretty flat frequency response. I like simple amps anyway.

I'm also looking at a 6-pos rotary switch with values from 1n, 1n5, 2n, 3n, 5n, and 10n as an alternative. It still gives me a knob to turn, as I like my knobs, but I also like to just set them. So that is a contradiction, I know.

Bass Controls.png

[bcmatt]

Hi Matt,

Yes, same layout as the Songwriter.

I've been doing some SPICE modeling of the SW bass control but in the context of a Hot Cat-like build, meaning, trying to create a sweep that is not too trebley or bassy and with a bit better response. Your work has confirmed that 1n5 is a comfortable spot for a single coupling cap value. The SW bass control produces a widely shelved response, rather than the classic -3dB drop of single, given coupling cap value. The SW also had the fixed treble peaker of 220p/330k which definitely bumps the treble up. The Hot Cat is more wide open but with a tight, controlled bass that starts to roll off around 300Hz. SPICE is showing that with 2n2 for the fixed capacitor and above that, in parallel, a 10n in series with 500k (maybe linear would be better) would create a similar roll off point, around 300Hz or so, like the fixed 1n5, and with the Bass knob turned up full, 7.2dB down to 50Hz, and turned all the way down, to about 3.7dB at 70Hz. Keeping the lower, fixed capacitor at a higher value (2n2 in place of the SW's 500p) keeps the gain in the midrange to low-mid frequencies. 500k does limit how much bass is rolled off, where 3M is going to give a deeper cut. When I was fiddling with a friend's SW30, his bass pot measured 3.7M but I didn't find what I was looking until after noon on the dial. The capacitance measured 8nF and above after that. About 2pm was the sweet spot (around 10nF or better). But again that amp had the treble peaker. We soldered in 2n2 on a switch over the treble peaker as a mid boost and it was better. So that tells me, I am going to prefer the more wide open nature of the Hot Cat's pretty flat frequency response. I like simple amps anyway.

I'm also looking at a 6-pos rotary switch with values from 1n, 1n5, 2n, 3n, 5n, and 10n as an alternative. It still gives me a knob to turn, as I like my knobs, but I also like to just set them. So that is a contradiction, I know.


Bass Controls.png

Thanks for this. Yes, I know what you mean. I go through the trouble of adding extra controls on an amp so that I can ideally never touch them again. I am starting to appreciate lately amps that strip away choices because they have been designed to stay in the sweet spots.

The Songwriter I didn't end up changing anything aside from removing the switch that would allow him to switch out of the Garter Bias. This is mostly because when I measured the bias at full VVR voltage with the Garter Circuit, it was already at about 100% dissipation. (Actually 98% and 102% respectively for each 6V6) They are not even remotely matched tubes... 2 completely different brands with 2 completely different lives... but probably both from the early 70s. I think I previously was trying to bias them conservatively as 14W 6V6GT tubes out of habit, and realized maybe they are regular 12W 6V6s. Anyways, I wanted him not to be afraid to use full VVR voltage if the moment arises (probably never will anyway). Anyways, he is happy with the current bass and cut controls, so I left it for now.

The fact that I already made a leather faceplate for my Hotcat makes me not very tempted to add un-needed extra tone controls for the clean channel. Although, I could in the future make the clean volume a push/pull pot if I ever felt like I wanted some sort of second choice. My 10W 360 Ohm bias resistors arrived yesterday and got them installed.

20250428_151547.jpg

With a bit higher mains voltage yesterday around 123.5VAC I was getting around 103% dissipation at full VVR Voltage. I assume this is OK?

My 6G3 (with 5E3 power section) is the one I am most interested in playing with some extra tone controls since I have to make a new faceplate anyways... but I really don't know if it needs it. It is also feeling rather balanced. I am contemplating whether I will even keep the MV before the PI (noon is the stock value) and the variable NFB (noon is the stock value as well). I'll try to play it a bunch more before deciding. I do have to say again though that converting the preamp and PI to 6G3 specs just makes it much more up my alley as an amp. Interestingly, my SSS is now the one I am least tempted to play live... and I gave it a lot of love dialing it in just over a year ago. Its complete lack of any compression is just too different than all my amps I'm used to playing. I think my next project needs to be a dumbleator to see what that does.

I think this Hot Cat with VVR and Garter Bias is finished for now though.

20250429_102238.jpg

Garter Hot Cat.png

[bcmatt]

Even though it is in a different thread:
https://ampgarage.com/forum/viewtopic.p ... 5&start=30

I should update this thread to show the current iteration of this amp:

Hot Cat with Elevated Heaters and Dumbleator.png

It now has an active FX loop (Martin's version of the Dumbleator) and elevated Heater Supply to bring voltages into a safer range for certain 12ax7s.

[bcmatt]

Probably the biggest thing that annoys me now about this amp is that anything above quite low on the treble control just sounds really bad... it's more like a fizz control as the gain gets higher... the ugliest form of fizz even though the "edge" control does some of that high end gain too (but I do find that one more useful to dial around a bit). The "treble" knob just gets turned way down permanently.
I notice that it is very much a vox tonestack and the same as channel 1 of the DC30:

Voxy Tonestack.png

I'm wondering if I should mess around with it. I don't want to lose the opportunity to remove the fizz, but maybe that can be done inside and then still be able to use the knob for something useful.
I notice the new Hot Cats have a different tonestack (along with many other big differences):

New Hot Cat Tonestack.png

My first thought is to lower the slope resister from 100K to the newer 56K... or maybe I'll put in a trimpot to be able to mess with it.
My next thought is to replace the 50p cap feeding the treble control with the newer 180p while also moving that 50p cap over to the output of the PI. Even though there is a cut control, maybe having the 50p parallel the whole cut circuit would keep the hash out if I am messing with the tonestack and removing the ability for the treble control to keep it out.

[bcmatt]

Using the tonestack calculator to see what's currently going on:
If my controls are at noon, it's probably doing something like this:

Hot Cat Standard Tonestack at noon.png

So, the controls are usually dialed in more like this:

HotCat Standard My Settings.png

If I lower the slope resister and increase the cap for the treble control, this should be the results at noon on the controls:

New Slope and Treble.png

Then pulling down both the controls a bit would look like this:

New sweep.png

Maybe a bit more useful? I don't know. I'm not familiar enough with what I like looks like in this circuit. I'm guessing that I'm trying to keep the 6-8K under control while still being able to dial in some bass and treble a bit. I could see that the treble control probably works more usefully as a balance between mids and useful treble.
I should be careful not to favour the new tonestack too much because the new Hot Cat has NFB and Presence, while my configuration has the classic Brilliance (cut) control, so I can cut more high end later in the power section while the new Hot cat can add presence.

But I do have that bright switch on the Loop Return if I need it. It's a pull switch on the loop Return which is now in the Master Volume panel position.

[Roe]

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?

neither, lift both ac wires instead

[bcmatt]

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?

neither, lift both ac wires instead

Is there a second-best option? The switch that fits in the front Panel is an SPST.

[Roe]

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?

neither, lift both ac wires instead

Is there a second-best option? The switch that fits in the front Panel is an SPST.

https://valvewizard.co.uk/standby.html

[Roe]

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?

neither, lift both ac wires instead

Is there a second-best option? The switch that fits in the front Panel is an SPST.

https://valvewizard.co.uk/standby.html