Hiwatt running HOT!

[alschnier]

hey guys,
I'm pretty green when it comes to amp repair, but am willing to jump in. I can solder & use a meter & have done some limited reading on amps.

I have a '79 Hiwatt 100 watt (2 input) that seems to be way underbiased.

I just took a reading, using an Amphead bias checker + DM. bias current ranges from 53 mA to 61 mA across the 4 tubes, w. plate voltage ranging from 610 - 632!!

this is in line w. symptoms I'd seen when I 1st got the amp. it shipped w. Groove Tubes (S-1), & they had turned a nice shade of dark brown. when powered up, the amp got physically hot & smelled like hot wax. the tubes were scalding & too hot to handle. the previous owner had this recently serviced & it was "fixed" by mounting a fan to the cabinet.

aside from 1 bulging cap, I don't see any red flags. the amp was modded @ some point, but seems to have been returned to stock circuit (unless something is amiss).

I already have a recap kit (from VHR), as well as a fresh set of tubes & plan to dive in next week. may also replace to the orange drops w. sozos. other than that, I have no plan (or clue) as to what the source of the problem may be.

any thoughts?

more pix here: Hiwatt FL112

 

[jay42]

any thoughts?

Check out the Hiwatt technical info...especially the bias mod, at www.mhuss.com

Tell us what the filament voltage is. 630V B+ is nuts. After looking at the badge, I believe this amp may have been built for the Japanese market. It would be fine in Europe, but not North America. R.G. Keen has a 'Vintage Voltage Adapter' box for bringing AC down for tweed era amps. It could be modified for this amp. www.geofex.com

What AC voltage taps does the amp provide?

 

[efnikbug]

Hiwatt = WhyHot?

 

[Jason_86_951]

Yep, voltage in Japan is 100 volts, you give it 110-127 from our wall sockets and you've got a high B+.

 

[alschnier]

Check out the Hiwatt technical info...especially the bias mod, at www.mhuss.com

Tell us what the filament voltage is. 630V B+ is nuts. After looking at the badge, I believe this amp may have been built for the Japanese market. It would be fine in Europe, but not North America. R.G. Keen has a 'Vintage Voltage Adapter' box for bringing AC down for tweed era amps. It could be modified for this amp. www.geofex.com

What AC voltage taps does the amp provide?

I've read thru mark's site again & again. spoke w. him about the bias mod, which may help, but even he was scratching his head @ such high plate V.

- how do I check filament V?
- when you ask about AC voltage taps, do you mean on the selector on back panel, or on the PT? (the panel selector range is 105, 115, 225, 245 & is currently set on 115v).

 

[jay42]

- how do I check filament V?
- when you ask about AC voltage taps, do you mean on the selector on back panel, or on the PT? (the panel selector range is 105, 115, 225, 245 & is currently set on 115v).

Check the AC voltage between pins 5 and 9 on the preamp tubes. Nominal is 6.3V. I doubt it's that low in your case.

Inspect the connections at the panel selector. If you've selected 115V, it's acting like it's mis-wired and you're really choosing the 105V. Do they look like they've ever been touched? Mark may have the color scheme somewhere...dunno.

Something you can do....remove all tubes and chart the AC voltages on the power transformer secondary. Do this for all the settings and report back. The 115 setting should be something on the order of 700Vac...(iirc, it's not center tapped on a 100W). Clearly it is higher.

 

[LPBlue]

This post made me think of something I had read in Aspen Pittman's Tube Amp Book which stated that a lot of those older Hiwatt amps didn't have enough bias supply to run properly. I don't have the book in front of me right now (I left it at work) but there is a fix described therein. Anyone else have the book handy that could quote for us?

J.R.

 

[jay42]

This post made me think of something I had read in Aspen Pittman's Tube Amp...

You don't need the book:

http://www.mhuss.com/Hiwatt/Schematics/BiasCkts2.gif

As you can see from the mods, they both need to be adjustable. The 100W circuit is the one that needs the voltage doubler to get a lot more range.