How Hot Should the Back Panel Get?

Bellycaster

I'm baffled as to why I've never noticed this, but the back panel of my Laney Lionheart 20 Head (4 x el84) was very hot after 2 hours use. Not the mesh metal bit, the part with the jacks in. Not sure if that's metal.

I know Valves get hot, but the back panel was almost as hot as a radiator. I had to take my fingers off after 5 seconds or so. I'm not sure if there is a fan in this amp. It cooled off after 10 minutes switched off.

Is this Normal in some amps? Ta!

ecc83

Check in a dim room that the OP valves are not 'Red Plating' The anodes should be a uniform slate grey with no hint of even a dull red on the centre. If no RP then your amp is probably fine. 30W 'class A' valve amps do run hotter than people expect!

Dave.

ICBM

Some amps do get very hot - it's not usually a fault, although it's not ideal since it shortens the life of some of the internal components, especially electrolytic caps.

Bellycaster

ecc83 said:

Check in a dim room that the OP valves are not 'Red Plating' The anodes should be a uniform slate grey with no hint of even a dull red on the centre. If no RP then your amp is probably fine. 30W 'class A' valve amps do run hotter than people expect!

Dave.

I might be getting something mixed up, but I though all valves had an orange-ish glow?

I have had a valve red plate on a previous amp and instantly knew about it. Can some amps do that without noise or smoke?

Thanks.

Bellycaster

I had to re-teach myself the anatomy of a valve. I was thinking of the the heater that must give the regular glow

I checked with the lights dimmed a fair bit and no "red" glow on the plate. I used the Blackstar JJN 20 tonight and for same period of time, it was only warmish to touch at the end. A lot less glass in that, though. 

ecc83

Yes, the heaters can distract you, that is why I said to look in a dim room. 'Mild' red plating will be just a slight red glow down the central spine of the anode highlighting the construction. That condidtion, though undesirable, is usually stable. More 'red' can lead to thermal runaway with the HT fuse blowing and destruction of the valves. Some brands of EL84 will RP where others will not. A small, 20% say, increase in the value of the cathode resistor can fix the problem permanently.

Do you mean the Blackstar HT-20? If so the first ones used EL34s in a combination of cathode and fixed bias and indeed were run well under max anode dissipation. Cannot understand WTF they went and changed it!

Dave.

Bellycaster

ecc83 said:

Yes, the heaters can distract you, that is why I said to look in a dim room. 'Mild' red plating will be just a slight red glow down the central spine of the anode highlighting the construction. That condidtion, though undesirable, is usually stable. More 'red' can lead to thermal runaway with the HT fuse blowing and destruction of the valves. Some brands of EL84 will RP where others will not. A small, 20% say, increase in the value of the cathode resistor can fix the problem permanently.

Do you mean the Blackstar HT-20? If so the first ones used EL34s in a combination of cathode and fixed bias and indeed were run well under max anode dissipation. Cannot understand WTF they went and changed it!

Dave.

Thanks. The Blackstar is a Jared Jamed Nichols sig amp head. It might be based on the HT20. I don't know a lot about Blackstar, though. It has 2 TAD EL84, The Laney has 4 Tads. Laney has more pre-amp valves, I think. Laney was hot enough on back chassis to only touch for 5 seconds and the Blackstar was only warm for same amount of playing time. Totally different beasts, though, I realise.

ecc83

Ah! Did not know of the JJN amp. Yes, just 2 EL84s and Blackstar will have made sure the valves are run inside their limits. Many of the "AC30" type circuits were slavish copies and took little account of 'tolerances'. Not only do valves vary a bit in their current 'pull' but you have to get the power transformer right! Mains voltage can be 10% above 230V (many of us still get 240V) and so that must be tested for and the traffs possibly changed (I did a LOT of transformer testing!)

Dave.

Bellycaster

ecc83 said:

Ah! Did not know of the JJN amp. Yes, just 2 EL84s and Blackstar will have made sure the valves are run inside their limits. Many of the "AC30" type circuits were slavish copies and took little account of 'tolerances'. Not only do valves vary a bit in their current 'pull' but you have to get the power transformer right! Mains voltage can be 10% above 230V (many of us still get 240V) and so that must be tested for and the traffs possibly changed (I did a LOT of transformer testing!)

Dave.

A daft question, but what are traffs? How does one measure the voltage coming from my mains. This amp goes to a few different venues for gigs so there's not much I can do really.

De_Batz

ecc83 said:

Ah! Did not know of the JJN amp. Yes, just 2 EL84s and Blackstar will have made sure the valves are run inside their limits. Many of the "AC30" type circuits were slavish copies and took little account of 'tolerances'. Not only do valves vary a bit in their current 'pull' but you have to get the power transformer right! Mains voltage can be 10% above 230V (many of us still get 240V) and so that must be tested for and the traffs possibly changed (I did a LOT of transformer testing!)

Dave.

The theory behind those Lionheart amps is that they run the EL84s parallel single-ended, which is why they only claim 20W out of a set of four. 
The big one runs 5 (yes, five) EL34s for 50W. 

Even just the heaters would be enough to keep my house warm. 

Jonathangus

Bellycaster said:

A daft question, but what are traffs? 

They're robots... in disguise.

ecc83

Jonathangus said:

Bellycaster said:

A daft question, but what are traffs? 

They're robots... in disguise.

Transformers. When I started in the R&TV trade they were always called "trannies" but that term has other connotations these days!

And I am no longer surprised at the daft way people build valve amps. There is NOTHING wrong with a pair of EL34 (6L6 if yer like) in push pull. Only want 20 watts? Ok then, knock the HT down a bit, bias to about 16W apiece and the buggers will last forever!

Dave.