vintageHIWATTconvention.com

hey folks.
the FL112 arrived today, safe & sound. now that I've got it in my hands, it appears there has been some internal work done (see pics). the amp actually sounds great. big & chimey w. tons of headroom. I popped in an EV SRO to demo it & it's a great fit (came unloaded).


the amp is running super hot, however. the prev. owner "fixed" this, by adding a fan ; ) it's running so hot, that the amp smells like hot wax, & the rear vent is pretty hot to the touch.

if it's a just a matter of being under biased, can I pop in some new tubes w/o burning them up?

* aside from the few changed caps, anything else stick out? (I did notice a few spots that were rewired. master vol. pot has been replaced).
* there are 2 add'l holes in the chassis leading me to believe this was once modded, & then undone (see upper LH corner preamp section. other hole was on rear)
* also, does this amp have the canadian mod (is that the yellow wire?) or need the de-fizz?


I'm not an amp tech, by any means, but not afraid to solder either. any help would be greatly appreciated.

more pix here: http://s86.photobucket.com/albums/k117/ ... wattFL112/

thx,
al

Sadly, photobucket and similar sites are blocked here at work, so I can't see your images.

You can easily diagnose if it's a PT problem by removing the output tubes and letting it run a while with just the preamps. If it still gets hot, you have a problem beyond just biasing. The PT should be cool to the touch when idling like this.

What kind of tubes are in there? Some modern EL34s need a higher bias than the stock Hylight amount.

--mark

thx mark. I'll give it a try. currently has groove tube S-1 power tubes (which, from the explanation on the GT site, sounds like the wrong end of the spectrum for a hiwatt). will try w/o them & LYK.

have a pr. of winged C's on order...

thx,
al

good news. no heat when I ran it w/o the power tubes. the only EL34s I have around are a pr. of old phillips. I'm going to try them for a few minutes, just to see...

alschnier wrote:* there are 2 add'l holes in the chassis leading me to believe this was once modded, & then undone (see upper LH corner preamp section. other hole was on rear)
* also, does this amp have the canadian mod (is that the yellow wire?) or need the de-fizz?

Yeah, it does appear to have been modded and de-modded. It looks like a stock 2-holer preamp now, albeit it a bit messy. And somebody swapped out a few mustards with orange drops as well. Oh, the humanity!

If you think it sounds spitty/splatty with the channels dimed, it might need a de-fizz -or- resistive mixer mod. Your call.

thx mikhail - I plan on returning the interior to its former glory, mustard camps, mil spec wiring etc.

- tried the other tubes (they were svetlana's, not phillips) & it got hot again. hmmm...

anything I can do while waiting for the winged C's to arrive?

upon further inspection, the green (plessy) filter cap has a slight bulge. perhaps this is the source of all my trouble? I'm going to recap to be sure.

would still like to get confirmation on exactly how to check the bias on the amp. I found (2) bias points on the schematic (between inputs & also near lone 47k diode). unsure where to go from here...

There is a good discussion of what, why, and how here:
http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#bias

Unfortunately, the stock 100 watt bias scheme is already pretty much maxed out in terms of what is available from the bias winding. If you need more bias (i.e. less idle current in the output tubes), you have to either (1) make a voltage doubler like shown on my site or (2) change the bias to the 50-watt type, which gets its AC from the HT winding, Marshall-style -- or find and use tubes that work with the stock lower bias.

(One of these days I'm going to do a write up on (2) as well.)

I really prefer to have the bias adjustable, as just because the plates on the output tubes are "not melting" does not mean the tube is biased optimally. I put a matched set of SED '=C=' EL34s in my #2130 and the bias is actually pretty close to right, but I can't speak to other brands as I have not measured them in an old Hylight.

--mark

thx mark. this is perfect. I'm going to take a reading, "before & after"

- "before": running in the red, w. the S-1 groove tubes, etc
- "after": recap + matched quad of =C='s, + replace a few user replaced elec. caps w. sozo's

after all is said & done, I may still opt for the bias mod. I just want to get it back to spec 1st, & then take a look. it needs the defizz mod, as well.

thx again. will post again next week once I cover some ground...

finally got around to taking some readings onthe FL112 "before" doing any work.

bias current ranges from 53 mA to 61 mA across the 4 tubes, w. plate voltage ranging from 610 - 632. clearly, this is running HOT. I picked up a recap kit & plan to jump in asap. hoping this + new tubes will help sort out the issue. anything else I should be looking at?

thx,
al

Did you perhaps make these readings on the AC setting of your meter? They are about 1.414 times too high.

If so, 43 mA ( 61mA / sqrt(2) ) is still a bit high. I like to bias these at around 35 mA.

--mark

I had it set on V in the "2" range. I'm using the Amp Head dual bias tester: http://www.amp-head.com/product_info.ph ... ucts_id=31, as I was still unclear on some of the other methods.

(according to the instructions, the tube cathode current is measured in DC voltage & a resistor is used to measure plate voltage by a reduced factor of 1000. advised to set meter in DCV 2 range).

it seems in line w. other symptoms - the current tubes show severe heat wear (groove tubes turned brown), amp runs very hot to the touch, smells like hot wax, etc.

I looked for the voltage doubler circuit on your site. is it the same thing as the "DIY 103 bias circuit"?

anything else I should be looking for? again, I'm hoping the recap will catch any errant components.

That bias tester still uses a meter to do the readout. The meter needs to be set to DC volts.

Yes, there's only one voltage doubler mod on my site. I've successfully added that to at least one vintage Hiwatt just adding parts to a lug strip attached to one of the can cap mounting screws. VHR is actually thinking about making this available as a mini kit item.

--mark

thx mark. I was set on DCV. just checked to be sure. it's a lot of plate voltage (& current). hope it's nothing serious (tranny).

Wow, that's crazy high voltage -- DR201 territory. The mains selector is set for 115v, right? Do you trust the meter? Grasping at straws here, but I can't think of any failure that would cause the B+ to go up 50%!

--mark