Re: Blackstar Artisan30 + Artisan15H

[DummyLoad]

- have one on the bench - output tubes red-plate. smell like something burned but not readily visible.

- i know, it's a shot in the dark, but anyone have a schematic?

- built in korea, P2P construction and very, very nicely done. mostly CC resistors and MPF caps. i suspect the OT has some shorted windings

- it's a cathode bias setup with 4 x EL84 and 68ohms under all of them - resistor measures good.  

- it appears to be similar to the matchless DC30 with some tone control enhancements.

- sent email to blackstar - no response yet.

--DL

[punkykatt]

Did you check the coupling caps from the PI for leaking DCV?

[DummyLoad]

Did you check the coupling caps from the PI for leaking DCV?

first thing i checked - not leaking.

[PRR]

> suspect the OT has some shorted windings

Would not cause red-plate at idle.

I suspect a bad tube. Since it's "all"(?), maybe grid-cathode short. Since it's P2P, change the 68 ohm 4X higher, say 330 ohm, stick EL84 in one-at-a-time, find the bad one.

[DummyLoad]

reverse engineered most of it... i'm sure that there may be a mistake or two... maybe. :p

output stage...
4 x EL84
1K screen resistors
cathode bias - 68R under all 4 tubes, bypassed w/ 470uF 63V

yada, yada, yada...

ALL PS caps 450V
all coupling caps 600V
all resistors 500mW CC UON.

[DummyLoad]

stick EL84 in one-at-a-time, find the bad one.

tested both sets of EL84s in known working maggotbox stereo amp this evening. B+ is 350 - all 8 tubes work.

this amp has other problems. again, i tested with two sets of tubes and it red-plates both sets. both sets were pulls from working amps.

according to the website, mfg. claims these have a 3year warranty. if it does, it's blackstars' problem. i'm near 100% certain this amp is less than 3 years old.

i reverse engineered it b/c when it works for a while it does sound really nice, so at this point i'm really more curious about what makes it tic.

at a glance;

other than the tube count it's not a matchless-DC30 clone - they omitted the CF in the triode channel and added a triode gain stage in front of the pentode. the contour network is different, pentode bias is much different, it has some funky feedback, and no master volume.

thanks PRR and punkcat.  

--DL

[VMS]

Maybe the bypass cap is faulty, try the amp without it.

Also, did you measure the voltage over the 68R resistor?


For the OT you could do the simple tests from this page:

http://www.geofex.com/ampdbug/outtrans.htm

[DummyLoad]

Maybe the bypass cap is faulty, try the amp without it.

Also, did you measure the voltage over the 68R resistor?


For the OT you could do the simple tests from this page:

http://www.geofex.com/ampdbug/outtrans.htm

VMS, thanks for the input, i plan to check the components of the output stage if i comes to that, however, i'm in a holding pattern until i find out weather or not the amp has a warranty so i haven't hit it with a soldering iron, yet. that will be the determination of my next course of action.

--DL

[DummyLoad]

no schematics for end users/repair shops. authorized warranty and repair stations only. typical. serviced and marketed in the USA by korg. UGH!

--DL

[DummyLoad]

ok, i got this thing up long enough to get some telemetry. installed a fresh set of sovtek tubes... sure enough, one starts to red-plate after few minutes.

345V to plates 340V to screens.

cathodes sitting at 12.0V

345Va-12Vk = Vak =333Vak

12V/68R=.176A /4 = 44mA/tube

44mA*333V = 14.7W Pdiss.

maybe we're a bit too hot?

Edit:

changed Rk to 82R 11W - Vrk now @ 12.7/82R = 155mA * 333Vak = 51W/4 = 12.9W/EL84 - much better.

B+ popped up a bit. amp will still red plate if output tubes are not matched with care.

amp has gone home.... owner left a darkstar artisan 15H for me to "look" at.   

i attached the latest schema - i took another couple of hours to look over the schematic confirming that i overlooked a few things 1) corrected input jacks - they were drawn wrong. 2) corrected the telemetry, and 3) added some service notes.

--DL

[DummyLoad]

same fellow that brought the artisan30 owns an artisan 15H - he let me borrow it to evaluate and reverse engineer. cool amp - better than the artisan30 IMO.

attached are the latest revisions for both amps in .SCH and .GIF formats.

--DL

edit: cleaned up artisan15 schematic - ignore V5 in filament chain.

--DL

[John]

Both saved to hard drive. Thanks for taking the time!

[LooseChange]

Put them in the Hoffman Schematic Archive!

[DummyLoad]

Put them in the Hoffman Schematic Archive!

if doug wants to then he can have them. otherwise get them now before the thread gets archived.

--DL

[PRR]

> red plate if output tubes are not matched with care.

It IS a bit reckless. That's good gee-tar amp tradition, but....

Go to separate 330r cathode resistors. Four 220uFd caps should work as good as the original large cap.

I suspect there is some other problem. Perhaps RF oscillation. Can you 'scope an EL84 grid? If it's redplating at the time, does touching the 'scope probe to grid change the redplating?

[DummyLoad]

Perhaps RF oscillation. Can you 'scope an EL84 grid?

not osc. in this case. i have a 100x probe - checked PS rail too - no osc.

--DL