Increasing 4 x KT88 bass amp screen resistors

[TDS]

I have built a 4 x KT88 turret board version of a Marshall VBA400. With the exception of the preamp heater supply (mine is 6.3V) and the DI output, it follows the Marshall schematic. I'm using actual VBA400 transformers and to compensate for only using 4 output tubes, my speaker output is 4/8 ohms instead of Marshalls 2/4 ohms.

The amp is killer and because of the B+ reserve, it more or less maintains 730 and 360V and with JJKT88's instead of 6550's starts to clip at just over 280 Watts.

The Marshall spec for the screen resistors is 270 ohms which struck me as quite low, but even after extensive bench testing, doesn't seem to cause any problems.

What has got me thinking is that the Trace Elliott V8 Bass amp seems at the other extreme with regard to screen resistors, they are rated at 2200 ohms, with a 10 ohm cathode resistor on each output tube.

Would I be correct to think that the V8 might have some degree of compression that might make it tonally different from the Marshall circuit which sounds fairly tight to me, although the oversized power supply in my amp may be part of this too.

I have never heard a Trace V8 and I'm wondering what difference to expect if I was to increase the screen resistors from 270 to 2000 ohms.

Any comments?

[Steve Conner]

Yes, what screen supply voltage do the two amps in question use? If the VBA400 runs the screens off that 360V, then the screen resistors don't need to be large, and the small resistors will mean screen voltage that sags less, which means less compression in the output stage and a harder, more punchy, dynamic sound.

If you increase the screen resistors to 2.2k, the power output will fall off drastically.

If the Trace runs its screens off a higher voltage, then the resistors need to be bigger so as not to overheat the screens, and that makes the tubes compress more.

We just had a thread on 4x 6550 amps with dual rail supplies.

[TDS]

Both the VBA400 and the Trace V8 have dual rail HT supplies with similer voltages (680/360), so the main differences, apart from the 10R cathode resistors (none used in the VBA) are the much larger screen resistors in the Trace. Both amps have the same output rating and I'm intrigued to try to understand why Trace opted for 2k2 instead of the more usual 1k in other KT88/6550 amps such as the Orange AD200B which is also a dual rail amp.

Was it possibly something in the tone they were looking for?

[mooreamps]

If you increase the screen resistors to 2.2k, the power output will fall off drastically.

No it won't.


-g

[Joey Voltage]

No it won't.


-g

Steve may have been referring to peak conditions, in which it depends on what the screens are actually drawing. In "normal" (whatever that means) conditions it probably wont matter much. Nothing I'm thinking too hard about at the moment though to be honest, so I'm willing to be swayed either way

[imaradiostar]

No it won't.


-g

dude, you say a lot of crazy stuff. Why?

As screen current goes up and voltage sags across the screen resistor, gain and power output drop. This is first year EE stuff, or at least teenager with a 50's tube textbook stuff.

I'm not sayin'...I'm just sayin.

just had to put that out there.

jamie

[TDS]

I'm getting 280W now with the 270R screen resistors and increasing them to 2k, I accept that the power would drop to some degree. Hovever, the Trace factory V8 test sheet shows that the octet of KT88's are expected to produce at least 400W with 2k2 screen resistors. With similer voltages, and screen resistors, my quad of KT's should therefore be able produce at least 200W.

I expect some degree of compression to take place, but I don't feel that the drop in power will be all that noticeable if I can still achieve at least 200W.

What I really would love to know is what difference to expect from the Trace arrangement with its 2k2 screen resistors compared to the Marshall arrangement with 270R resistors.

[Wilder Amplification]

I'm getting 280W now with the 270R screen resistors and increasing them to 2k, I accept that the power would drop to some degree. Hovever, the Trace factory V8 test sheet shows that the octet of KT88's are expected to produce at least 400W with 2k2 screen resistors. With similer voltages, and screen resistors, my quad of KT's should therefore be able produce at least 200W.

I expect some degree of compression to take place, but I don't feel that the drop in power will be all that noticeable if I can still achieve at least 200W.

What I really would love to know is what difference to expect from the Trace arrangement with its 2k2 screen resistors compared to the Marshall arrangement with 270R resistors.

Your power figures appear to be on the money. I think with added screen sag the amp will "warm up" a bit and while it more than likely will still be "tight", it won't be quite so tight and may actually add some missing character to it.

[Steve Conner]

OK, well here's one other consideration: Do the Trace and the Marshall use the same output transformer load impedance?

If the load impedance is higher, the tubes can make the same power with larger screen resistors, and indeed they need the larger screen resistors to stop the screens overheating.

With a low load impedance, you need smaller screen resistors to make the tubes kick out more current. There's no danger of overloading the screens because the heavily loaded plates suck up all of the electrons.

You can have the same power output in both cases. The high impedance case is more efficient, but the low impedance case may sound better, because the mix of distortion harmonics is different.

[TDS]

Missing character is an interesting way to look at it. I've wondered why almost every comment I've read in bass forums is very complimentary regarding the tone of the Trace V8. While the Marshall VBA has its following, it doesn't seem to have made the same impact even though its been in continuous production since around 1999.

Another interesting point for me is that the older Trace V4 (200W 4 x KT88) also seems sought after. It uses a different arrangement with a 22R screen resistor and a PTC resistor (value not known) on each KT88 screen. Both techniques (V8 & V4) are a bit off the mainstream, yet both seem to produce something appealling to bassists. Whatever the trace designers were shooting for, they seem to have hit the mark.

I can see that I am going to have to try bigger screen resistors in my amp to find out what the actual difference is. I've got some 2K 6W wirewounds and I may try them. At least I now have an idea as to what to expect. It comes down to what the actual difference is and whether it is noticeable in the tone.

[TDS]

OK, well here's one other consideration: Do the Trace and the Marshall use the same output transformer load impedance?

I don't know what the Trace's output impedance is, but I measured the Marshall OPT before I used it at 1.1K and intended for 8 x Svetlana 6550's (thats what the Marshall badged 6550's are), but I'm using 4 x JJ KT88's and (doubled the input/output impedances) therefore seeing 2k2 on a (now) a very oversized OPT into a 1 x 18 reflex cab fitted with an old Celestion K18J driver. It has no problem down to low B

I would be interested to know what the output Z was on the Trace, but there were not very many of them made, so the chances of getting the opportunity to measure an OPT is not likely

[pdf64]

My experience of large screen resistors, in regards of trying to reduce screen grid dissipation in the (6L6GC based) Fender 75s, is that larger value screen grid resistors (2k2, 3k3, 4k7) tend to make the shift the tone towards the thin and weedy side of the spectrum, more so the higher the value.
I even tried the thermistor method (I thought that I'd a novel idea with it) but they are too affected by the ambient temperature variation inside the chassis to give consistant performance.

[TDS]

My experience of large screen resistors, in regards of trying to reduce screen grid dissipation in the (6L6GC based) Fender 75s, is that larger value screen grid resistors (2k2, 3k3, 4k7) tend to make the shift the tone towards the thin and weedy side of the spectrum, more so the higher the value.

I even tried the thermistor method (I thought that I'd a novel idea with it) but they are too affected by the ambient temperature variation inside the chassis to give consistant performance.

This seems to show what happens if the screen grid resistors go beyond what is actually needed. In my case, there is no apparent screen grid dissipation issue. I was simply interested to understand why two amp manufacturers apparently chose different approaches for apparently similer output stages.

Your experiences with the thermistors may explain why Trace dropped the thermistor and resistor arrangement used in their V4 amps when they moved on to the V8 version.