Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TIMBO on July 10, 2015, 06:44:13 am
-
Hi guys, I couldn't pass this one up as it's OLD as and well used, BUT..........
The old carbon comp resistors (I'm assuming) have been replaced with carbon film and of course the large filter caps have been replaced.
SPOT THE MISTAKE COMPITION :wav:
I have checked my drawing a few times just to make sure I drew it as the chassis is laid out.
The person that can tell me WHAT the output tubes are gets the BIGGEST pat on the back as there is no markings on the tubes except maybe a "B" on one tube.
I can only say that these are the original tubes, I think :dontknow: Thanks
-
Those tubes *might* be 12BH7. Only say that because they've been in some old reel to reels I've bought. If they were the EL84's I bet you'd recognize those. If possible, measure the heater voltage on them. If it's 12 volts, it might be the BH7s.
Never mind! Just saw your drawing. So, I'm out. :laugh:
-
I fixed your drawing.
-
Ciao TIMBO
6BW6 ?
Franco
-
Thanks guys, I was pretty sure that the output tubes were EL84s as these were common in these low watt amps, it was just the way it had been rewired that through me.
Also V2 is not wired properly the .01 cap should be connected to pin 1.
My other thought is that the 100uf filter caps are a bit stiff for the 5Y3 rect tube as most had a 24uf in these positions.Thanks
-
EL84/6BQ5 if the OT is highish primary impedance (~7-10k). Or EL86/6CW5 if lower PP impedance (2-4k) and lower B+.
Any chance of a pdf schem?
-
I thought maybe - 6M5
Pin 1=g2,
Pin 2=g1,
Pin 3=k+g3,
Pin4,5 = Heater,
Pin 6=Internal connection,,
Pin 7=a,
Pin 8=Internal connection,
Pin 9=NC.
But, 6CW5 looks more like it, due to your unconnected pin one...
Pin 1=Do not use,
Pin 2=g1,
Pin 3=k+g3,
Pin4,5 = Heater,
Pin 6=Do not use,
Pin 7=a,
Pin 8=Do not use,
Pin 9=g2
......
Yes, 6BQ5/EL84, same pinout as 6CW5.
-
Any chance of a pdf schem?
MATON B4769 - TIMBO
(http://i.imgur.com/EBA3l6p.jpg)
MATON B4769 - SLUCKEY
(http://i.imgur.com/dg8XE1L.jpg)
Ciao
Franco
-
Thanks K
-
tim, if it were mine, i'd very likely mod it. mod shchematic attached.
for more vox-like tone, ditch the NFB.
--pete
-
tim, if it were mine, i'd very likely mod it. mod shchematic attached.
for more vox-like tone, ditch the NFB.
--pete
You allus gotta hack away at things! :icon_biggrin:
-
The original schematic ( if correct ) would create the worst signal-to-noise ratio possible. The only signal attenuation is prior to the first stage, and the first preamp valve is an EF86 - always problematic for noise and microphonics. It appears to be an el-cheapo way to have two inputs with separate gain controls, without using an additional valve section.
There is a possible problem with Dummyload's modified schematic. It places the tone control inside the negative feedback loop, which may cause instability at some tone control settings.
-
TIMBO have you measured the OT to establish the primary impedance ?
Franco
-
Great! I like it. Be interesting where you go with this one :dontknow: Platefire
-
The original schematic ( if correct ) would create the worst signal-to-noise ratio possible. The only signal attenuation is prior to the first stage, and the first preamp valve is an EF86 - always problematic for noise and microphonics. It appears to be an el-cheapo way to have two inputs with separate gain controls, without using an additional valve section.
There is a possible problem with Dummyload's modified schematic. It places the tone control inside the negative feedback loop, which may cause instability at some tone control settings.
as already stated. ditch the NFB.
--pete
-
Thanks guys, As suggested some mods done to bring it up to scratch.
I've kept the R and C values the same as presented, changed the first and second filter caps to 30/22uf as I would think that the originals were probably 24/24uf as this was quite common.
I changed some of the carbon film resistors to CC resistors as this also would have been the originals.
Knowing that the CC resistors were not great in the power supply these will remain CF resistors.
With the b+ at 389v the cathode resistor on the EL84 is up to 220r/5w and suppling 11w/tube, I may increase this value as I am unsure of the speaker output, I have yet to remove the speaker to clean and repair if required.
The .47uf/270v filter cap of node D worries me a bit, although it does measure .48uf so i'll take that as its still working.
The component values around the EF86 are going to be taken as the same as the originals, as I have nothing the compare them with.
The injecting of life..........
All good.
Voltages taken and they seem to be in order.
The amp sounds fine, with a little HUM and some static type background sound.
With guitar plugged in a very warm and a creamy tone that has a bit of punch.
To my surprise the controls are responsive, with the tone working well.
The FB is connected and I will try it disconnected.
As usual the EF86 IS micro phonic and when tapped rings through the amp, I replaced it with a new tube and it was the same.
I still have to clean the rust off the chassis as well as the usual socket maintenance.
Some guidance on the EF86 and the .47uf on the power supply. thanks
-
DL, The tone control is wired before the EF86 this way there is only one wire to connect it to the Ef86, if it was down stream there would have to be an extra two wire back up to the control panel.
It sounds good as is with good tonal coverage. thanks
-
The .47uf/270v filter cap of node D worries me a bit
Since your node D only feeds one stage you may be OK with that small cap. I ran into the same thing when I built my Lil Maggie (http://home.comcast.net/~seluckey/amps/lil_maggie/Magnatone_M2.pdf) amp. It used a .1µF to filter node D. But my node D was feeding three consecutive triodes! The amp would motorboat at higher volumes. I put in a 47µF to cure the problem and never looked back.
-
Thanks Sluckey, I subbed the .47 with a 10uf and couldn't hear any difference, not that I though I would.
I have a .47 mustard that I think i'll used.
I pulled the speaker and the cockies haven't kind and have had a nibble. I found a replacement on ebay so if I don't win it i'll patch the old one.
-
Hi guys, New/old speaker found to replace old one.
Some new tolex as the old stuff was painted black and it started to lift off the backing cloth.
Did a CRAP on the corners of the tolex so corner guards added.
A bunch of new screws.
I could not find grill cloth similar but found a old Stromberg Carlson Stereogram that kindly donated some neat valves and transformers as well as a neat piece of grill cloth that looked very similar to the original.
The EF86 is a little ringie when tapped but seems to be OK.
It has some low level HUM but this does not vary when the volume is increased.
Some hash and fizz that is ok ( after all it is 56yo, found date stamped on PT).
It had some feedback type sound (feedback resistor removed) so I added the .01 snubbers between the plate and screen supply and this seems to have fixed that.
Sounds pretty good with plenty of tone through the range of the tone pot and also plenty of volume in the volume pots. These are a bit starchy but trying to leave it as stock as possible.
Just one thing.....
I removed the FB resistor from where it connected and I would think that it would have had only one resistor on the cathode, is there any ratio to the cathode resistor to plate to maybe give me a ballpark value that would be close to the original value.
I looked at some od the vox values and they are much different, ideas. Thanks
-
Looks good, what did you use to clean the chassis ?
-
Hi kde, The chassis had some surface rust on top.
I just scraped the rust off and gave it a good wire brush then a couple of coats of silver rust guard, should hold it for a few more years. :icon_biggrin:
-
> removed the FB resistor from where it connected and I would think that it would have had only one resistor on the cathode
If you disconnected the 56K, and left the 2.2K, you are golden.
Formerly you really had (2.2K||56K)= 2.1K. Maybe that was right, maybe it just came-out that way with nearest standard values. IAC, 2.2K==2.1K for any practical vacuum-tube purpose.
-
Thanks PRR. :icon_biggrin: