Broken Hiwatt... Suggestions?

[Dashface]

So, I've got a Hiwatt Custom 50 head. It's a Biacrown head, from about '82. It's in good shape externally, but I've never opened it up (only had it about two months now).

At practice the other day, the normal channel started humming _very_ loudly. I assume it's not the tubes since the bright channel still works just fine. Anyone have any idea what might have gone wrong? The input jack doesn't move at all, so I'm assuming it's not that.

[hamfist]

I'm not familiar with the layout of the amp but it's quite possible, even probable, that the normal channel uses a different tube or "half tube" (as a 12ax7 is in fact two separate circuits) to the bright channel. So could still easily be a faulty tube.

[gldtp99]

Hamfist probably is correct---- it could likely be a preamp tube that's gone bad (or half a 12ax7 since the Bright channel is ok).
This Biacrown Hiwatt preamp uses 1/2 of V1(1st gain stage) and also 1/2 of V2 (tube mixer stage) for each channel, if this is the standard Custom 50 DR504 and not the "OL" higher gain model which uses a resistive, not tube, mixer---- if you do have an "OL" then just replace V1.
So either the V1 or V2 tube could be the culprit---- replace both of these tubes first---the problem could be something else but odds are it's one or both of these tubes...................gldtp99

[Dashface]

Thanks fellas - I've got a set of new power tubes to go in (which haven't yet cause I'll have to get that bastard re-biased), but I haven't swapped out the pre-amp tubes yet.

Should I stick with 12ax7? I want as much headroom as possible.

[RedMan]

I would also check the ground on that channels input jack.

[zzmoore]

.
Should I stick with 12ax7? I want as much headroom as possible.

Kind of an Oxymoron there. That is to say, Hiwatts are nothing if not clean headroom. As others have said, try some new 12AX7's first. Get the amp running the way it should, then look into a 5751 to see if that will give you more RPM's. We have a 1979 DR50. It wears 12AX7's in the same spots as yours. Can't imagine it having more headroom.
Good Luck :AOK

[callaway_1]

for headroom try a CV4024 in the PI.
It will never have as much headroom as a 4 input HIWATT, but the CV4024 will help some.

[Dashface]

Is the CV4024 an at7? Should I stick with ax7s for the rest of the preamp tubes?

[gldtp99]

This particular Hiwatt curcuit is designed to use all 12ax7's in the preamp---- some earlier Hiwatt circuits use three 12ax7's and one 12at7 (in the PI).
They've already suggested that you try a hi-quality 12at7-type in the PI and a 5751 in V1 (i think) for more headroom--- i think you should run all 12ax7's as my very similar Hylight 2-input DR504 has plenty of clean headroom with all 12ax7's--- and i like the 'feel' of 12ax7's over the lower gain tubes in most cases.
You can run 12ax7, 5751, 12at7, 12ay7, 12au7 in any or all of the preamp positions without hurting the amp in any way--- it's all up to how you think it sounds best, there's no "wrong" way to do it if you like the results.
I like all (hi-quality) 12ax7's in this particular Hiwatt circuit---- you might like something else--- Experiment--- Have Fun--- Trust Your Own Ears !!!............gldtp99

[Groovey Records]

Just drop it off at my place and I'll fix it right up for you in a couple of decades

NLT end of the century

[Dashface]

Well, the 12ax7 in the V1 position was in fact toast... It was blackened, which could explain things. Luckily for me, it was a cheapie Mesa/Boogie tube.

The nice thing was that the other preamp tubes turned out to be vintage Mullards! So, I just put a JJ AT7 in the PI, and kept the mullards in everything else (V1 through 3). Sounds pretty damned okay!

I also got some new EH big bottled 6CA7s that I heard were great in Hiwatts... But I'm afraid to put them in there without biasing. I swapped 'em in for a few minutes, but took 'em out again because they were getting _really_ hot. The tubes that are in there at the moment are really old and have no labels left... But they are KT77s. I'm assuming I've gotta find an amp guy in town to futz with the bias. Or I could just keep 'em - they are rare-ish :)

[gldtp99]

I'm glad the preamp situation has worked out ok---- Mullards !!!
Now be sure to run properly biased output tubes and the Biacrown will sound great and likely last a good while................gldtp99

[sliberty]

If V1 had a Mesa 12AX7, it might be that this is the second time that V1 went bad. Perhaps the Mesa tube was the replacement for the first time. You might check to see if the components connected to V1 look OK. Perhaps the plate and / or bias resistors should be replaced (or at least checked). In fact, until you are sure all is well, maybe you should put the JJ in V1 to protect your nice old Mullard from going bad like the previous two V1 tubes did.

[Dashface]

Ah - you've got a good point there, sliberty! Though I think it could just be a ruse by everyone to have me open the amp up again so I can take pictures haha :D

Honestly, I really do need to suck it up and have a tech put in some bias trim pots or something... Problem is, of course, that there aren't any amp techs anywhere near where I am at all.