what 6BM8 (=ECL82) amps do you know?

[telegumbo]

hi,
i got myself an old amp combo with 2x 6BM8 amp 1x 12AX7.
it says "made in italy" and according to its looks and the dates on the caps, its from the mid-seventies.
it has two input jacks and controls for vol, bass, treble, tremolo speed, tremolo intensity.

does anybody have an amp with a comparable setup (from other makes)?
is it common for this tube setup to get quite hot?
as i understand, the tremolo is driven by one of the 6BM8 tubes (at least it flickers to the speed of the tremolo).
if so, is there just one 6BM8 used as a power tube?
how many watts does this produce?

however, it sounds really great but its also very "fragile".
recently it started to add crackling noises to the otherwise cool sound.
the bad noises seem to increase with higher volume and tremolo intensity.
maybe caused by vibration - they also appear when you knock on the amp.
is anybody familiar with that problem?

thanks.

 

[bunny 7]

This is a modern amp using 6bm8 tubes.
Looks interesting.

(link removed)

 

[printer2]

It is a push-pull amp, someone was looking for information on it some time back and I posed a link to the schematic. I can't seem to find it now without the name. I think it is about 8W. The 6BM8 has a triode and an output pentode in the same tube. Probably a similar setup is the Hagstrom 39. The tremolo is on the 12AX7 rather than the 6BM8.

 

[jefrs]

Description sounds like a copy of the 10W Westminster

Watkins/WEM Westminster
http://web.archive.org/web/20061127...n.com/bargainbin/wem_westminster_mkIX_10w.pdf
and
WEM Dominator "25 Mk.II Bass" 1xECC83 2xECL82

The ECL82 is a triode-pentode, in a p-p pair the two triodes are used as the PI for the pentodes.

The ECL82 is a "hi-fi" valve, hence used in the Bass version of the Dominator instead of the usual EL84 pairing - the actual WEM Dominator "25" (guitar) from the same 70s period is 3xECC83 2xEL84 (one ECC83 for each channel, one ECC83 for PI).

Nominal output of p-p ECL82 is 10 watts. But guitar amps usually have them running over their max rated HT of 300V (a Brimar can run to Va=900V) Wa (absolute max) is 7W each) so you could be looking at 20 to 25 watts tops (rate speaker accordingly).
Normal rating for a pair of ECL82 is only 10 watts.

If you have a tremolo channel then that could be arranged as clean on one side of the PI and tremolo on the other, earlier versions of the Dominator with the EL84s did this.


The crackling could be old bottles, they are not fragile like some valves are, but they could be worn out, they can go microphonic (the innards are, complicated).

The flickering is not a good sign, that shows the valve is drawing a lot of current.
You may have old and bad electrolytic capacitors. Can you check the HT supply rail for ripple (loud mains hum too)? - if not then test the capacitors, failing that then just change them.
You can see the valves? - ECL82 normally have shrouds.

Do check the seat connections are good - remove and re-seat the valves

Good NOS ECL82 are still available - Brimar, Mullard, Mazda(France), Philips(Holland). RFT, Telefunken - roughly in that order
New? - maybe the Svetlana ECL82/6MB8

ECC83? - the world is your lobster: Sovtek, JJ, several flavours.
Ime don't bother with NOS, you cannot get Brimar or Mullard and the best new ones are better than the remaining NOS.

 

[telegumbo]

Wow, thanks a lot guys.

The crackling could be old bottles, they are not fragile like some valves are, but they could be worn out, they can go microphonic (the innards are, complicated).

I've already changed the old Phillips ECL82 (made in GB) for some cheap NOS ones (SB Elektronik, West Germany - Maker unknown:
As I found out, they just put their label on tubes from Eastern Europe, but did not produce them theirselves).
However it didn't change the faulty sound.

12AX7 is still old. I'll change this one next.
I have one 12AT7 laying around. What do you think, can I safely replace it with that?

The flickering is not a good sign, that shows the valve is drawing a lot of current.

ok, but couldn't this be normal since the tremolo has no switch (only two pots) - so it's always kinda "on" even when it's off?
Note: The tube flickering equals the set tremolo speed.

You may have old and bad electrolytic capacitors. Can you check the HT supply rail for ripple (loud mains hum too)? - if not then test the capacitors, failing that then just change them.

Yes, there is loud mains hum too. But i can't really tell if it's a normal amount.

You can see the valves? - ECL82 normally have shrouds.

Sorry, I don't know the meaning of this. If that's some kind of fixation device,
no it does't have anything like that. The tubes are just put in their sockets with no extra holding device.

Do check the seat connections are good - remove and re-seat the valves

I'll check all that +report back.


THANKS (so far)

 

[printer2]

The Westminster has a Long Tail pair phase inverter which needs two triodes leaving no triode for the tremolo. Your amp uses a cathodyne from what I remember. Not sure if the ECL82 was designed for hifi, more like a one tube solution for cheap record players, add a rectifier tube and you are done. looked at the data sheet, 10-12W seems reasonable.

You could swap in the 12AT7, not optimum but good enough to check things out.

Another amp with the same tube compliment, Gibson GA-8t.

http://www.diyaudio.com/forums/atta...mp-se-pp-gibson-ga-8t-amplifier-schematic.pdf

 

[jefrs]

Have you found a make and model to your Italian amp?
To mess abut with an old amp the diagram is almost essential.
With a simple amp like this it should be possible to reverse-engineer one. I have done that and can assure you it is a complete pita.

"Shroud" is a sprung metal sleeve around the valve.
It has an earth connection to ground. But it means you can only see the tops of the bottle.
My ECL82 Bass amp has shrouds on the ECC83 and only one of the ECL82.
If I put the shroud on the wrong bottle I get an unearthly loud banshee howl.

The Westminster never had a tremolo, you need to look at the early Dominator circuits for that. The later Dominator lost the tremolo too. But it used an ECC83 for the tremolo. The ECL82 are PI and output, and they should not flicker. You could, as Printer2 says, be using cathodyne driver using one of the triodes and the other for the tremolo.
http://web.archive.org/web/20071101...cheaven.com/bargainbin/wem_dominator_1965.pdf

The reason I opine on the 50s WEM amps is because so many 60s Italian electronics were based on British stuff. I'm surprised at the Hagstrom, ultra-linear is a hi-fi thing that rarely sounds good with guitar (because we push the gain up into distortion)

You should save these diagrams because SchematicHeaven is extinct and we have no idea how long Wayback can keep them available

The OP valves should light up (quite spectacular) on output surges like a big power chord. Not on and off like a faulty neon sign, doing that will wear the valve out (it splatters the anode all over the glass).

A 12AT7 won't have the gain needed for the pre-amp, ECC83 are cheap enough.
You might use the 12AT7 it you want to blow the amp's brains out with a harmonica.
I have tried a Mullard ECC81 (12AT7) in amps and it just makes them bland and lifeless.
The ECL82 triode is a line driver, so you are getting the extra current an ECC81 would give in the PI. The pentode section of the ECL82 requires more shove than an EL84.
A 7025-STR is an ECC83 with a slightly different flavour.
I prefer a "low-noise" ECC83 in V1 such as http://www.watfordvalves.com/product_detail.asp?id=694
But this one may benefit from having other high gain stages following as in bigger amps.
The Harma Retro are good and I prefer the sound of the cheaper Sovtek to the "better" Electro-Harmoix.

It was the EL84 that was devised cheap and nasty with extra gain for picnic box record players and wireless sets. The ECL82 doesn't work well as a one-bottle solution, not enough gain in the triode (it's a line driver). And it does have a more level response making it suitable for what was then considered high fidelity - the dreaded radiogram.

Eastern Europe produced some excellent ECC83, notably the bloody good East German RFT ECC83. There was a Mullard/Philips licensed company in Yugoslavia too (EIA?) that made good bottles. They keep rumoring to re-start production with their genuine Mullard tooling, but nothing happens.

The mains hum is worrying because my WEM Bass ECL82 amp does not hum.
Loud hum I associate with the electrolytic caps going home.
This could be main cause of the flickering.
I know I sometimes bang on about electrolytic caps lasting for centuries, they can and then they can go down, but I can test their operation ...
So if you don't have test gear, change the caps as a matter of course, they are 40+yo - time for them to retire

 

[telegumbo]

The amp make is "Sound" or "Sound Italy" -this is engraved on front and backside.
The model name could be "B20" -this is written on front with black tape (!)
I don't know if somebody invented that name or wanted to re-write the model name.
I didn't try to remove it yet to see whats under it.

When i get back to my amp next week, i'll check the things you suggested.

Thanks.

 

[printer2]

Triode in the ECL82 has a gain of 70.

Vintage record player uses an ECL82 3 W single tube as amplifier.

http://www.youtube.com/watch?v=eNmE7pe5VJ4&feature=plcp

The ECL82 is an audio triode plus output pentode. The ECL82 was much used in record players with crystal pickups of the late 1950s and early 1960s. Crystal pickups had a relatively high output and when coupled with the single ended pentode output stage made effectively a single valve design. Only a rectifier was required to complete the line-up.

http://www.r-type.org/exhib/aai0261.htm

NOVAK 291A AMPLI-STEREO ECL82 TURNTABLE AMPLIFIER 1959

 

[Mat]

Try the (Gibson) Maestro GA-1RT and GA-1RVT.

From memory these both use 6BM8's and sound fantastic. I have owned both and currently have a GA-1RVT which is a brilliant little conbo woith reverb and tremolo.

The schematics are avialable on the web.

Good luck with the project

 

[Rich_S]

Marshall also made an ECL82 amp, the 10-watt "Popular" model. Similar to the 18 Watter.

My DIY "18 Watter" is built with ECL86's, which are similar to the ECL82, but have a 12AX7-equivalent triode with a gain of 100. My "13 Watter" has a pair of ECL86s that comprise the phase inverter and the output stage. One more 12AX7 make sup the preamp.

Sadly, the ELC82 and ECL86 are not pin-compatible.

 

[Tony474]

The Westminster never had a tremolo...

Actually, Jeff, just irrelevant nitpicking, but some of the original Westminsters did. Have a look here . The maroon-and-cream example I owned around 1960/61 didn't, though. However, I have no doubt that your technical info is spot-on and useful for the OP to know.

 

[backalleyblues]

Another amp to use 6BM8s was the Univox U-45B... it also has a tremolo circuit (mine doesn't work) the Unibox mentioned above is a modern re-creation of this amp circuit. Tube lineup was 6X4 (rectifier) 12AX7, 2x 6BM8, produced maybe 10 watts on a good day, and had some the gnarliest distortion when cranked... this amp really sounds like no other amp, it really is it's own thing...

The chassis was MIJ, wonder if the Italians decided to copy this amp? That would be pretty funny...

Franc Robert

 

[jefrs]

Marshall also made an ECL82 amp, the 10-watt "Popular" model. Similar to the 18 Watter.

My DIY "18 Watter" is built with ECL86's, which are similar to the ECL82, but have a 12AX7-equivalent triode with a gain of 100. My "13 Watter" has a pair of ECL86s that comprise the phase inverter and the output stage. One more 12AX7 make sup the preamp.

Sadly, the ELC82 and ECL86 are not pin-compatible.

Marshall copied both the 10W WEM Westminster and the 17W WEM Dominator.
The 18W Marshal is a dead knock-off of the 17W Dominator.
However both WEM amps were based on Mullard Circuit Book standard amps, so no infringement.

The ECL82 normally runs at 250V (as 10W Westminster) but my WEM bass amp is 300V. Wa was/is 7W (that's when the anode melts). The current Svetlana 6MB8 will go to at least 600V and Wa=7W, so output similar to an EL84 is entirely possible, but Svetlana says their 6MB8 output is 1.8W, not 3.4W as the ECL82 is, makes me think they are not completely identical bottles.

None of the ECL 80/82/83/86 have compatible pin-outs. Perhaps just as well because their characteristics are quite different.
The pentode section of the ECL82 wants a lot more current to drive it than an EL84, an ECC81 might manage that but not an ECC83

When I was in my teens I had a side-line of repairing picnic box record players. I saw several with single valves, but never an ECL82.
I have an ECL86 in an old tape recorder.

 

[dickiebyrd]

Another amp to use 6BM8s was the Univox U-45B... it also has a tremolo circuit (mine doesn't work) the Unibox mentioned above is a modern re-creation of this amp circuit. Tube lineup was 6X4 (rectifier) 12AX7, 2x 6BM8, produced maybe 10 watts on a good day, and had some the gnarliest distortion when cranked... this amp really sounds like no other amp, it really is it's own thing...

The chassis was MIJ, wonder if the Italians decided to copy this amp? That would be pretty funny...

Franc Robert

My Unibox is an interpretation of the U45B - the main difference is I use the triodes in the 6BM8s as a long-tail pair (if I remember right the original was split-load) to facilitate the same bias-vary tremolo and reverb that are in all of my other amps - I use 2 12AX7s; half of one is identical to the U45B pre (grounded cathode, 4.7meg grid leak, etc.). The other half of V1 is reverb return and V2 is reverb send/trem osc. The original had a one-knob trem and was not ever available with reverb. With new production Electro-Harmonix tubes and a 5-volt rectifier like 5Y3, together with a PT secondary of 600vct (original 500 or 520) I get an even 10 watts with a 1kHz tone at the onset of distortion.

I have made the same amp using ECL86s (saved from Hammond organs) which are pretty much a half 12AX7/EL84 in the same bottle - and accordingly, you get 16-17 watts, biased exactly the same as '84s. But NOS ECL86s are next to impossible to find, even though they were the last new original tube design well into the transistor age... if anybody knows where I can find some...

 

[jefrs]

ECL86?
http://www.watfordvalves.com/products.asp?search=ecl86
British made Mazda are almost-Brimar and the AEG /are/ Telefunken
Two of the best then. Not cheap!

 

[dickiebyrd]

ECL86?
http://www.watfordvalves.com/products.asp?search=ecl86
British made Mazda are almost-Brimar and the AEG /are/ Telefunken
Two of the best then. Not cheap!

Thanks - I've never been to that site - I don't know the current dollar/pound conversion rate, but I would imagine that by the time they got here, they would each be around $60USD, which is not terrible - not cheap, but attainable...

 

[telegumbo]

I've disassembled the amp now +ordered some new parts, mostly small caps.

There are also two big aluminium housed caps which i want to replace:

1x Ducati Axelyt 32+32uF 250/300V
it has one ground pole and then there are two other poles which are soldered together -does that make sense?

1x Ducati Fixelyt
[Circle Symbol] 100uF / 350V
[Triangle Symbol] 100uF / 350V
This has also 1+2 Poles and theres a separate wire from every pole.
The replacement I've ordered has yellow and red pole markings.
Anyone know how to match this with my triangle and circle symbols?
Do I have to match it at all?

Then there are some yellow foil caps which I want to replace:
0.015 and 0.012 /400V.
The replacements are only available with 0.010 uF.
Without knowing the scheme +based on gut feeling:
What do you think, can I safely use those or should i rather get the ugly modern ones with correct values?

Then there are two strange looking resistors (completely coated? in red / green).
What is it? Does an exact replacement make sense or can I just use standard ones?
(all other ones in this amp are standard type, too).

By the way, I mentioned above you can't switch the tremolo.
Well you can - by footswitch. I completely ignored that because I never tried it out.
How does that usually work? when you switch it off and remove the footswitch, does it stay off?
or is it always on, as soon you remove it?

thanks.

 

[valiant]

Interesting thread and where else other than TDPRI can you find Mr Sessionette and Mr Goodsell having a chat on the way to Watford for valves....anyone know the UK price of the 6BM8 Unibox....there must be a market for such an amp in the UK with it's classic psychedelic R&B/garage punk sound....all hail the mighty 6BM8.

 

[Tony474]

Bit off-topic, but Watford Valves are a very good company indeed. I've dealt with them on a couple of occasions and, while they normally supply only by mail order, as I'm local I've been able to pop round to their nice place in Bricket Wood and collect in person. Highly recommended.