As some folks know I am somewhat of a skeptic regarding the use of closely-matched power tubes in a guitar amp. For hi-fi, it's a different story of course. Matched tubes weren't used in the good old days our favourite vintage amps were built. They just pulled the next two tubes out of the box and plugged them in. Furthermore, a degree of imbalance in the PI and power amp helps allow some even order harmonics to be produced, giving that classic tube sweetness to the tone. I've read in a few places, although I have no idea how true it may be, that the fad for using matched power amp tubes in guitar amps was something started by Groove Tubes as a way for GT to make more money.
So we're only using a single bias pot. And in our bench and studio tests, we haven't been using matched pairs of 6V6s or KT66s either. They were always a few mA out in every case, and the discrepancy also varied with B+ voltage. So far there haven't been a lot of complaints about the tone.
rjgtr wrote:
I really like the Hiwatt output stage for it's toughness, even when clean. That's not to say I prefer it to my normally more Marshall style EL34 output stage, but it is cool. I'll be interested in the schematics for the TRIWATT. It sounds like a cool amp.
The TRIWATT uses the early 70s style preamp, with the CF and 12AT7 PI tube.We found the tone got tougher (and louder) when we swapped out the 12AT7 for a 12AX7. Looking at the various Hiwatt BBS's there seem to be fans of the mellower early 70s 12AT7 tone, as well as the late 70s/early80s 12AX7 tone.
On the subject of the Hiwatt vs Marshall power amps, earlier in this thread I was asking Tripower455, who you may remember from 18watt.com, about his Jimmy Page DR clone, coincidentally also named "Triwatt".
Tripower455 wrote:
I call it the Triwatt JP504! It's a Jimmy Page/SAP preamp mated to a DR504 power section, with a switchable choke. .... It sounds awesome... With the choke engaged, it'll do the Marshally thing, and with the 100R, it does the 2 hole Hiwatt thing. I prefer it this way. Oh, and unlike most Hiwatt inspired amps, it sounds best with Celestions!
zaphod wrote:
I've been looking for some time for someone who could do a direct comparison of the Hiwatt tone with and without a choke. So you say it sounds more Marshally with the choke in - meaning like... does it break up sooner... have more grind... or....??? Is it more/less punchy with the choke? I'd love to know.
Tripower455 wrote:
It breaks up a little sooner and has a noticeably softer attack. It is overall a much looser feel than the using the 100R resistor. I like it, but it really isn't my cup of tea for everything or my style of playing. I prefer the tighter, punchier Hiwatt type output section. It's neat to have both!
So it appears that the choke is a major factor in the difference between the feel of the Hiwatt and Marshall power amps. Without the choke, then those huge filter caps make the power amp real stiff.
Our TRIWATT's power stage is virtually identical to the stock Hiwatt's, except the 100 ohm power supply ballast resistor has been changed to a 2k5, in order to get the screen voltage down to a safer level for the 6V6s. Generally, I've stayed very, very close to the way Hiwatt did things.
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