Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on April 15, 2017, 05:17:03 pm
-
Hi there.
I got this amp last week, and after be really afraid about inner voltages (you can check the whole process here; http://el34world.com/Forum/index.php?topic=21818.0 (http://el34world.com/Forum/index.php?topic=21818.0) ), i finished the schematic.
Amp was made by a Barcelona company here in Spain maybe in mid 70s.
The company is Centro Musical Barcelona (CMB) and still is in business but i think they just not build amps anymore.
Design and look is deeply influenced by early Orange and Matamp designs.
-
some pics
-
more
-
David, are you considering subtle improvements to the amp, after checking all the parts are ok (especially coupling cap leakage)?
Such powerful amps can benefit from little practicalities like adding EL34 cathode current sense resistors, and removing/bypassing switches and poor speaker sockets that can accidently disconnect the speaker. Elevating the 12AX7 heater may also reduce stress on the phase inverter triode.
Ciao, Tim
-
hi !
in the "other" topic you said:
"There are often dropper and stopper resistors in the screen circuit, especially for quads. Also you didn't indicate if the measured voltages were with the output stage operating at nominal idle power dissipation (and what that level was for each EL34), and if the 'screen voltage' was at the power supply or on the actual screen when in idle. The PP stage output transformer impedance (assuming you are using matched speaker impedance for the transformer) also indicates, when looking at loadline, how much the amp will push in to overdrive and how heavily the screen current may rise (and hence the screen voltage may sag)."
I´m still working on the amp.
Just made some fire up test (with bulb limiter first) and nothing blows, but amp need some work before is in useful condition.
What you mean for?
"....indicate if the measured voltages were with the output stage operating at nominal idle power dissipation."
My english and my tube knowledge is limited.
-
Hi David,
The power supply voltages depend on how much current is being supplied.
You identified power supply voltages in the other thread, and then effectively concluded they were too high, and hence changes to the power supply would be needed. That may have been a poor initial judgment.
A more considered assessment would be to restore the amp, confirming that no parts are poor, and the idle setup has appropriate EL34 power dissipation, and to measure power supply voltages, and confirm cathode and screen currents. Then you would be in a better position to check the EL34 loadline, and how the original designer set the amp up. You may want to operate the amp for a particular use (PA, guitar, hi-fi), which may then lead you to question if any changes are needed.
Ciao, Tim
-
Ok.
I have no intention to use the amp until i change some stuff.
Diodes, bias supply, E-caps....
I´ll come back with more information when amp is in better shape.
Thanks!
-
Hi,
I´m measuring O.T. plate to plate impedance.
I applied 232vac into both primary sides and get 15.4vac on the 8 ohm sec.
so...
232/15.4 = 15.065
15.065 x 15.065 = 226.95
226.95 x 8 = 1.815.
So.
Is this a O.T. wired in 1k8 configuration?
-
1.8K per four tubes (3.6K per pair) is what Marshall used.
Do any Marshall "100W" use screen voltage this high?
What is your heater AC voltage? If it is much over 6.3V, then wall outlet voltages have risen since this thing was made. You could connect a 240V:24V 1A transformer to buck-down wall voltage 10%, say from 245V to 220V, which may be what it needs.
-
Heaters are 6.47vac for the 12ax7 and 6.3vac for the EL34.
-
In 70/80s we had here in spain 220vac.
Now, the standard is 230vac.
Heaters seems to be ok at 6.3v ans 6.47v.
What make me suspicious about something going wrong are the e-caps in preamp power supply nodes running way above its ratting voltage.
They seems original to me. If they had been running 40 years 100v over its voltage capability would be a surprise. Something is wrong here....
Anyway, amp need some work.
Diodes in rectifier are from diferent model an age.
A 10k resistor in power supply is measuring 15k and looks a bit burned.
I think a total recap is a must.
Some hours before any other voltage check.
-
What make me suspicious about something going wrong are the e-caps in preamp power supply nodes running way above its ratting voltage.
Did you have all the tubes, especially the power tubes installed when you measured the voltages on the preamp e-caps? If not, all readings on your schematic will be high.
-
Yes. All tubes in
-
Well then, I'd be concerned with 519v on a cap rated for 450v.
-
Me too.
All the in the amp is oversized. It weird to me that the builder use those caps. Something more is wrong here
-
I remembered seeing this somewhere and found it, it's just an other VVR but seems more for screens, there's a data sheet but I didn't look over it:
https://www.tube-town.net/ttstore/Kits/Power-Supply/Kit-TT-VoCoM-Variable-Voltage-Regulator::5651.html (https://www.tube-town.net/ttstore/Kits/Power-Supply/Kit-TT-VoCoM-Variable-Voltage-Regulator::5651.html)
https://www.tube-town.net/cms/?DIY/Amp-Tools/VoCo_Variable_Voltage_Control (https://www.tube-town.net/cms/?DIY/Amp-Tools/VoCo_Variable_Voltage_Control)
as for the over all voltage, I too thought you had the tubes out when measuring. It's cooking everything, even the preamp tubes are seriously high. It's not a vintage collectible, unless maybe in Spain. Maybe get a new PT for it? Might be tough to find something ready made with the doubler but look at Marshall or Hiwatt PTs, I believe Majestic in UK does one-offs, is familiar w/ guitar amps, and should be able to help, or maybe a local winder. Maybe you can spec something wound to drop in with little modification to the chassis, just enlarge the screw holes a bit. You could also rebuild the PS w/o using a doubler making it easier to find a PT. 4X EL34s is lot for everything these days especially at 600V, maybe rebuilding the power supply to give you 370-430V would be more useful - though I have to admit that >500V is part of that Orange sound.
BTW are those Partridge transformers or just Spanish copies - Perdiz? The OT looks like a C-core - pretty serious iron.
-
hi!
Yes, i saw that kit and people here have issues about the 600v ratting for the mosfet and the pot handling those voltages.
And... yes. tubes was all in when i measured the voltages.
I just ordered some stuff yesterday and next week i´ll do some work.
New rectifier diodes, new E-caps (rated for real voltages in the amp), check the bias supply, power resistors, leaking caps, grounds....
And new tubes, of course.
When amp arrive i check the bias: 33-40-32-64 mA. Far away from a matched quad.
Before going into any other conclusion, i´m gonna do all the basic job.
After that, new voltages measurements and, if needed, look for a solution.
Right now, and after some research, i found all this options. All of them had been tested in working amps:
.-VVR for the screens. Tube town kit or any other with better voltage specs.
.-Using the central point of the 2nd power supply node as screen supply. (Half voltage, maybe too low)
.-Add some zener diodes in line with the screens, before the 1k screen resistors. I have some 20v/5w. Five of them and i am in the safe zone.
.-Doubling O.T. primary impedance. If i use a 16ohm speaker with the labeled 8ohm tap i get 3k6. Could be a solution? PLEASE. SOMEONE CHECK THIS.
Or, just rise a bit the screen resistors, set a moderate bias level and not worry about all that any more.... :icon_biggrin:
-
1.8K per four tubes (3.6K per pair) is what Marshall used.
Do any Marshall "100W" use screen voltage this high?
What is your heater AC voltage? If it is much over 6.3V, then wall outlet voltages have risen since this thing was made. You could connect a 240V:24V 1A transformer to buck-down wall voltage 10%, say from 245V to 220V, which may be what it needs.
checking Hammond specs. i found this.
(http://www.trinityamps.com/ForumGallery/trinity/Tube_impedance_chart.jpg)
They ask for 3k4 for 4xEL34. So... if i use the "8ohm" output with a 16ohm speaker i´m in the 3k6 area. Ballpark??
-
Tubes can be run with a WIDE range of loads, depending on the B+ you use and your output goal. The Hammond chart is only a suggestion, and probably not a good one.
Find a Marshall, Orange, or Matamp which is similar and study what they did.
-
With voltages over 500vdc with 4xEL34
Matamp uses 2k5 pri.
Laney Supergroup MK1 uses a 3k5 (600vdc!!!)
Marshall goes from 1k7 to 2k
Park used 2k5