Kt88 - El34

Theres a certain someone on ebay who sells 'affordable' valve amps who is selling the same amp as mine; AI VR70. He has a few different variants of the amp, none of which impress me (compared to my mods ) but he has used KT88's on one. Whats actually needed to allow my EL34 PP to use KT88's? Biasing is very easy so thats not prob, any ideas.

PS. Sorry to refer to the above person

 

i think.... that it's only biasing that is needed.

 

I think....you maybe right. I cant find anything else that needs to alter and Ive got plent of volts to play with (not literally, that would be silly).

 

It is only biasing but you may not be able to use them right out of the box. Is your amp self-biasing or manual? You may have to adjust the cathode bias resistor to allow you to be in the correct range. I don't know your amp though so its just speculation

 

Its manual, and its a doddle. Ive been having a very quick flick round every so often and its looks like KT88's tend to be biased around 490V. Im going to have to have a go with a set!

 

Does the bias let you get up that high though?

 

If not they are easy enough to sell on, might even buy em off you myself as spares dpending on which brand you bought

 

Yep, Ive seen 600V on the DVM when I was abit heavy-handed with a trim pot.

 

The 490 Volts is the high voltage rail , the bias voltage , is the negative voltage that exist between grid and cathode. Is that negative voltage , that define the bias current.

Jorge

 

T-bone,

What do you think of the standard VR-70? The Naims have just sold so its decision time.


Paul.

 

Hmm slightly confused here;

I normally take my bias from the bias test point which measures the voltage across a 10 ohm resistor, cathode to ground. I just apply ohms law to work it back. I have been in the amp many many times and take measurements from the pins after Ive done a mod, I always get the same results either way.

The 490V is what Ive worked out the bias needs to be for KT88's using a GE spec sheet and working on the 70% rule. Since then Ive done abit more working out and have come up with the following;

max dissipation - 35 watts, volts at anode 600V (I will measure this when I actually change the tubes). 35/600 = 60mA. 60mA x 70% = 42mA thus, using ohms law I need to read 420V for the correct bias. Like I said, these are merely early workings just to get an idea of what I need.

 

don't forget may be teaching a granny to suck eggs but in doh!!! just realised its fixed bias, excuse me, my mistake, this means volts at cathode are fairly neglible, but its anode MINUS cathode volts TIMES current. I was thinking cathode bias.

I know he's not a popular guy, but £250 for a valve amp and warrantee isn't to be sniffed at.

Its the same circuit as WADs el84 if I am correct, ecf 82 pentode/triode into concertina phase splitter.

The Affordable valve el34 one, made by V.A.L in china has 3 pots inside, so makes it a bit tougher to work out which one does what.

Is yours much easier...could we have an internal pic?

Your calcs look right and that's the easiest way to measure bias. Apparently you can go down to 50% on the icon audio (el34)which I have but that's a bit cool, I haven't the facilities to check if that will cause distortion. It sounds good though at low bias.

Apparently looking at cattylink, VAL have stopped making valve amps, so I don't know what he who must not be named is going to do when his stock runs out.

ps kt88 take more heater current, but I think 4 of them only make aoubt 10% more IIRC so that's not really significant heating of the mains tx when there is 1.6 amps? through each valve? And you never really use them at max power, too.

 

VFM is dam good, right from the off ot kicked my marantz 6010KI square in the nuts, it really was love at first listen. I havent seen the german guy selling then recently, the cheapest Ive seen is £250 + delivery which still pretty good. One problem you may have is with the single input, this means a seperate input box. I couldnt find anything commercially available that I liked so I made my own for about £40. The standard EL34's are okay but nothing special, a set of Svets made a big improvement. The splitter/driver tubes are pretty decent, I have about 4 pairs and theyre about the second best (or they were, theyre quite fragile ) so you can stay with them. The only fault I found with mine was poor grounding on the input phono's so I simply changed them for decent pair. A big plus point is the lovely large output trannies.

Since then Ive played and tweaked and modded and now have a dam fine amp that I have no desire to part with. I also think the amp is a great starting point if you like to fiddle, the circuit is amazingly simple, biasing is a breeze and theres plenty of space to work in.

One final issue you may have is with the standard volume pot. This is 20k which means that its very sensitive and fractions are only needed to adjust the volume. This resistence might also cause some probs with phono amps. I changed mine for a Blue Alps 50K which is fine BUT not as good as the original IMO.

So, basically for the money its pretty dam fine and I have listened to many, many amps. Its got me hooked on valves to the point were I find SS amps uncomfortable to listen too after an hour or so. For the money you cant loose.

Another fan: www.apah69.dsl.pipex.com

 

Precisely...and why not to measure the bias current , only in the voltage that it imprint in the 10 ohms resistor and leave alone the high voltage supply.

As you have find 42 mA current ( a conservative value , because you must take screen grid current in account ) it represents 0,420 mV across the 10 Ohms cathode resistor.Is easier and safest to measure...


Jorge

 

Obviously you must read 0,420 Volts...

Jorge

 

Thank you for sharing your knowledge.

 

Whoa dudes and I mean WHOA!! Pretty much everything that has been said in this thread about biasing valve amps is not just wrong it's spectacularly wrong!!

It has nothing to do with anode Volts! One person has it basically right up thread....

The bias is the NEGATIVE Voltage relative to ground present on the grids of the output valves. This sets a specific current through the valve which, when there is provision to measure it, is usually read by measuring the Voltage drop across a very small (1 Ohm often) resistor between cathode and ground. When there are bias test points provided, and you are measuring usually a Voltage in the region of 10 to 70mV, then this is what you are measuring. Note that with tetrodes and pentodes this figure will include the screen grid current. Hence the negative Voltage at the grid is the bias, which sets the current, and you are measuring this current via the Voltage drop present across a cathode resistor to ground. This resistor must be as low a value as possible otherwise it will cause unwanted negative feedback and reduce power.

Rather confusingly this adjustable biasing arrangement is called FIXED BIAS, as it is theoretically fixed at what you set it to.

Some vintage amps and a very few modern ones have no bias testing resistor at the cathode and you simply set the negative Volts at the grid to that specified by the manufacturer. This not good as it fails to take account of the characteristics of the actual samples of valve fitted.

The other main system is known as cathode bias AKA self bias and as this has no adjustment provision I'll leave it there on that one.

Fixed bias can give more power from a given set of valves than cathode bias FWIW.

 

The circuit you described with the ecf 82 pentode/triode into a concertina phase splitter is intriguing. As an amateur concertina player, I appreciate the craftsmanship that goes into these circuits.