Question: Is a 30-watt amp head going into a 2x12 cab good enough for medium-sized gigs?
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Old Music Guy
Member
I live in west central New Hampshire, about 500 yards from Vermont. With my 40W Blues Deluxe, I can play along with bands playing on the other side of the river, and be heard. Luckily, they haven't tracked me down yet 
Track Rat
Just Your Average Sized Member
I suddenly like youI live in west central New Hampshire, about 500 yards from Vermont. With my 40W Blues Deluxe, I can play along with bands playing on the other side of the river, and be heard. Luckily, they haven't tracked me down yet
Old Music Guy
Member
Took long enough
Old Music Guy
Member
I like that track! Well done.
I am not in the LEAST surprised! That Blues DL is a very different beast from the 30 watt AC30 type amplifier (which itself is 'king loud!) It runs two 6L6 OP valves at over 400 volts in "fixed bias". Thus the 40W rating is almost certainly its "clean" power output and distortion would be well under 10%. Pushed hard I would expect that amp to put out close to 100 watts.I live in west central New Hampshire, about 500 yards from Vermont. With my 40W Blues Deluxe, I can play along with bands playing on the other side of the river, and be heard. Luckily, they haven't tracked me down yet
Always remember that the 'advertised' rating of an amp is, or should be that power it delivers below clipping, distorted power can be much higher depending on the circuit configuration. Factories should also rate amps at the local 'nominal' mains voltage. 230V in UK and EU but most of us get 240V +
Dave.
Old Music Guy
Member
I remember reading that the Blues DL is the same circuitry that was in the old Bassman 100W amps. I have not verified it, but I would believe it.
Interestingly, the amp came with 2 GT 6L6-R output tubes. One died. For my use I couldn't justify the $$$ for a matched pair of GLs. so I put a matched pair of JJ 6L6s. I was blown away by the reduction in the output with the JJs vs the GTs. It was immediately noticeable. For my use, it works, but if your gigging? Go with the GTs.
It is my understanding that Groove Tube valves are good quality components that are selected into bias 'batches'. Therefore the set in your BDL will have been chosen to give optimum results, probably a bias current of 25-30mA per valve.
My guess is that the JJs you fitted needed a lower bias voltage and were running very 'cold'. Unfortunately the schematic I find for that amp shows no means of bias adjustment. Should not be an expensive job to have done though.
Dave.
Its not really a tube brand more than finding ones with a certain gain characteristic. Which is always been the downside to a non-adjustable fixed bias circuit. Just like Mesa Boogie amps. They market a set of tubes for them they selected so that they would work with that bias voltage. So when they sold that amp they engineered it so no tubes but their own that they sell work correctly unless you go through trail and error on your own with different tube sets.
If I had that amp I would just mod it for adjustable bias so I can stick whatever tube I want in the output circuit.
Pretty much d', the world and his wife hates paying for service people and GT built their business on the idea that you did not need to go amp-in-hand to a technician just to change and bias your OP valves. You do pay over the odds for GT valves however!
But yes, one 6L6 is exactly the same as any other 6L6 (or it should not be called a 6l6!) except that there is a range of grid voltages for any given anode current (at a specified anode and G2 voltage) Once biased to that point the valve will perform to the data sheet spec'. That bias variation has become larger in the last 50 years as valve production has mainly gone to 'lesser' shall we say sources. I found at least one 100W EL34 amp where I had to mod the bias circuit to get more range to get some samples of OP valve to the specified 25mA. 'It'appen and with valves it can only get worse and vastly more expensive!
Modding the existing bias circuit to make it adjustable is not difficult for someone that can solder. The main thing is working safely and the circuit must be configured to 'fail safe'. that is, if the preset resistor wiper goes open circuit the bias voltage must shoot to max and not zero.
Dave.
But yes, one 6L6 is exactly the same as any other 6L6 (or it should not be called a 6l6!) except that there is a range of grid voltages for any given anode current (at a specified anode and G2 voltage) Once biased to that point the valve will perform to the data sheet spec'. That bias variation has become larger in the last 50 years as valve production has mainly gone to 'lesser' shall we say sources. I found at least one 100W EL34 amp where I had to mod the bias circuit to get more range to get some samples of OP valve to the specified 25mA. 'It'appen and with valves it can only get worse and vastly more expensive!
Modding the existing bias circuit to make it adjustable is not difficult for someone that can solder. The main thing is working safely and the circuit must be configured to 'fail safe'. that is, if the preset resistor wiper goes open circuit the bias voltage must shoot to max and not zero.
Dave.
LazerBeakShiek
Rad Racing Team
honestly how long do your valves last? Years..Or stay in spec? I kinda like the modded bias tones you can get playing around with it.
Yeah, Ok ..Once adjusted sure. New.
My EL34 has new EHX tubes and it biased out nice and proper. Power amp kicks ass on everything.
LazerBeakShiek
Rad Racing Team
Dave, I got a marshall EL 84 20/20 , with a UK transformer. Is there a cross-reference part that is US wall voltage you might know of? It is a 5 wire transformer.
So I dont need that step up converter box.
So I dont need that step up converter box.
I don't know details on Marshall amps, only what I can glean from online schematics and they rarely show traff taps!
Make yourself known over at <thefretboard.co.uk> there is a retired amp tech there with the handle ICBM and he knows EVERYTHING. Nice guys too, nuts about guitars and shit but nice guys.
Power valve life? Phoar! How long ha... OK! When I was a radio tech we serviced some old valve PA amps. These used KT66s and were fixed biased and rated at 50W. They lived in cupboards in WMClubs and got used once a week for bingo but often got left on for days. They kept working for 20 years without a hitch.
The same goes for hi fi amps (though the 'tweaks' probably changed valves every year at least!) a decade no problem.
It was not that these valves did not run hot, they did, especially the hi fi jobs, close to max dissipation and cathode biased. They had a long life because they rarely got driven into hard distortion and so did not suffer kV 'flyback' spikes that tended to destroy screen grids. By contrast, the valves in a TV WERE constantly run 'king hot AND had kV pulses on their electrodes. Two years for a PL509 line OP valve was good going.
Also, PA and hi fi amps used a fair bit of Negative Feedback and that kept the performance stable until the last knockings. Gitamps use little or no NFB so once the 'shine' goes off a valve you tend to notice.
Dave.
Reply from the top man there..."No, I don't think so. The schematic shows a single winding and lists three separate part numbers for the PT for 100, 115 and 230V. " So it would be a new traff and fork knows how much that would cost you.
I actually agree with the design practice. That is to supply amplifiers with the correct transformer for their market country.
Dave.
I actually agree with the design practice. That is to supply amplifiers with the correct transformer for their market country.
Dave.
LazerBeakShiek
Rad Racing Team
But this has 84's. UK gets the goods this time. They are so sweet..use a tiny 6 or 8" celestion or orange speakers..put it in your bedroom..not go deaf..
Old Music Guy
Member
I thank you, sincerely, for that information. You sent me back to my books. I'm an analog guy, but not a tube person per se.
I had a Dynaco ST 70. I had replaced the original RCA unmatched tubes with a quad set of Mallards. Part of the installation was adjusting the bias. Never thought that this would be an issue with this amp, but after digging in, I realize that a bias adjustment *should* be in order. I had completely forgotten that the grid bias need be adjusted for each tube set.
Until I actually open the amp up again I won't know for certain. But the videos show that there is a pot for adjusting the grid bias.
That said, how do I know, or find out, what the correct voltage(current) is for the JJs?
Thanks in advance
PS: On my Dynaco, It specified what the bias voltage should be: 1.342V. And that measurement was across a resistor. (I think 1 ohm, but that seems like a lot of current)
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Ah, that opens a whole new can of worms! The schematic I found is probably an older one, perhaps they put bias pots in the later models. Giving you a value for the bias current is the easy bit. I biased all EL34S, 6L6, KT88 at 25mA. Some folks like to go hotter but you need to watch the anode dissipation, 25W for the 6L6 so for an anode supply of say 430 V 25mA will give you 0.025X 430= 10.75W . Some folks say biasing hotter makes the amp sound "better" but I have never seen any evidence for that. Wasting power and reducing efficiency offends my engineering sensibilities! In any case, amps get hot ENOUGH and heat is the enemy of all components. Hobby horse stabled.
You need to be sure you can work safely on high voltage circuits. If so I can give you chapter and verse on setting the bias. A schematic of that actual amp would be a great start but a really good gutshot the next best thing.
Dave.
Old Music Guy
Member
That is my first rule of life. 30kV DC was my worst bite. I still have the scar. Your advice is sound, and well taken.
What I've read today with regard to the grid being hot or cold, In a "passive" context, a cold grid would be analogous to "a limiter", i.e., resisting the flow of electrons from the cathode to the anode, and vice versa. The hotter the grid, the higher the gain/output.
That was the number that I've seen floating around for the 6L6.
I really appreciate your responses, and digging in. I am much obliged. I come from an analogue background, but op-aps and transistors. So, I appreciate the lessons on tubes.
You are very welcome but I am not sure? Do you want to go ahead and re bias the amp? If so PM me then we can communicate via email. I do not post potentially lethal methodologies on an open forum...some daft ***er will have a go and die!
Dave.