Restoring a Beck Amplication Model 4 PA

a fine line between stupid and clever

Postby sorenj07 » Mon Jul 31, 2006 10:00 am

any pictures? it's a place to start.
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Postby erichayes » Mon Jul 31, 2006 3:28 pm

Hi All,

A few comments, suggestions and guesses . . .

Mullard and Telefunken ECC83s are famous for what you describe at cold turn-on. Since this amp is British, the '83s are probably Mullard and what you're seeing is normal.

The delayed pilot light is probably because it's a neon bulb being run off the B+ in the power supply. You didn't mention what type of rectification is used, but I'll bet there's a GZ34 in there. Since it has controlled warmup, you won't see any high voltage for a few seconds after turn-on. That' would also explain why the pilot light stays on after shutdown; the HT caps are still charged and thus able to supply power to the lamp. As far as the tubes staying lit after shutdown, that's what tubes do. It takes time for them to warm up; it takes time for them to cool down.

For some reason, guitar and PA amp manufacturers like to push EL34s closer to their limits than Hi Fi amp makers, and it's not uncommon to see signs of arcing on their sockets in those types. The best way to eliminate the problem is to use either ceramic or mica filled sockets as replacements.

There are two basic kinds of hum in amplifiers: ripple and audio. Ripple is characterized by having only a fundamental frequency (in your case, 100~) and nothing else. It's indicative of either open filter caps or excessive current draw--sometimes a combination of the two, but not often. It can be verified by putting an AC voltmeter across the filter caps and noting the voltage. I consider 5 VAC on an input cap to be the upper limit. Caps further downstream should measure in millivolts.

Audio hum, on the other hand, has both a fundamental frequency (50~ for you) and lots of fruity harmonics. It's usually found in higher gain stages and can be caused by anything from heater-to-cathode leakage in a tube to a ground loop or high resistance ground. It's the nemesis of amplifier designers, but if you assume that it wasn't there to begin with, you can usually find the cause and eleminate it.

There are a few things I make my interns commit to memory. One of them is this:

ALL OCTAL OUTPUTS EXCEPT 7591 AND 7355 THAT DON'T HAVE PLATE CAPS:

pins 2, 7 = heater
pin 3 = plate
pin 4 = screen grid
pin 5 = control grid
pin 8 = cathode
pin 1 = suppressor grid (EL34 and KT77)
pin 6 = tie point

9 PIN ECC** SERIES:

pin 1 = plate 2
pin 2 = grid 2
pin 3 = cathode 2
pins 4,5 = heater
pin 6 = plate 1
pin 7 = grid 1
pin 8 = cathode 1
pin 9 = either heater center tap or internal shield

EL84:

pin 2 = control grid
pin 3 = cathode
pins 4,5 = heater
pin 7 = plate
pin 9 = screen grid

Spending time to memorize those pinouts can be as helpful as memorizing the color code, because you'll be able to look at a tube socket and know what it's telling you without needing a schematic. If pin 8 on your 6L6s are connected to ground either directly or through a real low resistance, you'll know you have fixed bias and you know that the voltage on pin 5 needs to be negative and if it's not, you know there's a bias problem. If you go to pin 1 of your ECC83 and find 23 VDC on it, you know you'd better check the other end of the resistor or wire that's connected to it to see what voltage is there, 'cause 23 volts ain't right.

Let us know what you find.
Last edited by erichayes on Mon Jul 31, 2006 9:18 pm, edited 1 time in total.
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Postby erichayes » Tue Aug 01, 2006 12:20 am

Hi All,

tmod, I'm going to suggest some things to check in a post after your reply to this one (and I assume others will chime in as well), but I need to know what test equipment you have available. My suggestions will be based directly on the equipment you have, or have access to, so don't leave anything out.

I'm half way through the 1930s amp restoration, so my juices are really flowing in this area of restore, rehab, and revive. I am, however, going to be taking my reproduction systems on the road in a couple of weeks to exhibit on the dog-and-pony show circuit and probably won't have reliable access to a server. So, please, let the information flow liquidly in the interim.
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Postby erichayes » Tue Aug 01, 2006 10:28 pm

Hi All,

I'll bet there are a lot of guys reading this who are jealous of the stuff you have, which covers about 90% of the problems likely to be encountered. You didn't mention if you had a variable AC source like a Variac, but at this stage, it's not that critical.

I want to say before starting that I'm not being intentionally patronizing when I "Heathkit" a procedure. If you know how to set your test gear up, do it. There are some folks out there who might have similar equipment that they've been afraid to use; the step-by-step is to assist them.

First thing is to try to locate the source of the hum. Put a dummy load on the output (8Ω resistor or 20W 12V automotive lamp), short the inputs, hook the scope to the output (make sure you've located the trace first), set the scope to 5mS/div and turn the amp on. After 10-15 seconds you should be able to find the hum on the scope screen by varying the vertical sensitivity. What you'll see could be a really dirty sine wave, a sawtooth wave, or some combination of the two. If it's predominantly sawtooth, go directly to the filter caps and replace them.

Ascertain if the heaters on the preamp/mixer amp tubes are running on AC or DC. Go to pins 4 and 5 of one of the ECC83s with your DMM set to AC volts. (If 4 and 5 are connected together go between them and pin 9.) You should see either roughly 6 or 12 volts or less than .5 volts. If it's one of the former, the tubes are AC heated; if it's the latter, switch the DMM to DC volts and check the reading for 6 or 12 VDC more or less. If it's signifcantly lower, and/or if the AC voltage is over .5, look for a high capacitance, low voltage electrolytic (usually no less than 500 µF @ 50 V or less) and replace it.

If the heaters are AC operated, remove the shields, if any, and grab each tube with your fingers while watching the scope. If the hum level doesn't change, or changes uniformly for each tube, go to the next paragraph. If one tube has more hum than the others, pull it (yes, while the amp's on) and check the scope. If the hum drops substantially, that's your problem tube, or stage. Try substituting another tube and see what happens.

If no one tube stands out, remove the preamp tubes one by one while watching the scope as outlined above. Handle the same way as above.

If the amp still hums with no preamp tubes, move on to the tone control amp stage. Fortunately, design usually follows function, even in England, so the TC amp should be close to the tone controls.

Keep proceeding from stage to stage until the hum diminishes. If it doesn't by the time you get to the output stage . . . well, tomorrow's another day.

Caveat: I've been doing this professionally since 1967, and a lot of my technique has become second nature to me. I try to be as thorough as possible in explaining my procedures, but some things fall through the cracks. If you have questions, please ask. " 'Tis better to ask stupid questions than to make stupid mistakes."
Last edited by erichayes on Thu Oct 12, 2006 1:07 am, edited 1 time in total.
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Re: Restoring a Beck Amplication Model 4 PA

Postby Shannon Parks » Wed Aug 02, 2006 6:24 am

tmod wrote:The amp is made by Beck Amplification (anyone got any info on the company?) in England.


Interesting. There is <very> little info out there on this company. Here is an ad from the early 70's:

Image

A poster on a guitar forum claims it was actually a company formed by none other than Jeff Beck.

Shannon
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Postby dhuebert » Mon Aug 14, 2006 9:45 am

From what I've learned alot of this old equipment came with factory installed buzz (the buzz goes in before the name goes on). Caps were expensive and people didn't really know better so a little buzzing was tolerated. I have modified some old guitar amps and made them perfectly quiet.

I don't know if you want to make upgrades but here goes:
The first best way I have found to improve an old amp is to provide a separate ground for the pre-amp circuit. Everything up to the phase invertor (but not including) should be star grounded to the input jack. Next, disconnect the preamp power circuits from the can caps. Install new caps (40uF) right at the preamp tubes, grounded to the star ground you've already made. Parallel those spare can caps for the power amp section. Ditch those old diode rectifiers for UF4007's. Look at your power amp grounds, is anything grounded to or close to the power transformer? Stray electric fields will generate eddy currents in the chassis which can induce hum into circuits incorrectly grounded. Don't be afraid to put a three prong plug on and ground the chassis or replace every electrolytic in the box. None of this stuff should change the way the amp sounds.

Just about any schematic will do as it is unlikely the amp has any unusual topography, most of this stuff came out of the RCA receiving tube manual anyway. I've had good luck finding schematics at duncanamps.com

Don
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Postby Clanger » Wed Feb 07, 2007 3:18 pm

Hi ,

I just joined this forum this evening, to post some photos of a mullard 5-20 I am working on in the eiclone pages.

Just trolling around I noticed this item. I can tell you that those amps were made in Wellingborough, Nothampton by a chap called Derek Tompkins. He engineered at a quirky studio there of the same name. He built a very good sounding valve mixing desk for the studio amongst other items. One of the most famous bands to record there was indie band Bauhaus in the early 1980's.


I haven't spoken to him for several years, but I should imagine he's still alive and kicking.

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