vintageHIWATTconvention.com

This is my Hiwatt Dr103 of 1971

https://picasaweb.google.com/1178923843 ... ZHNpqmi0QE

what do you think about that? Is it all original?

I don't understand how to change the Power Tubes without set the bias, there are some kit already matched for this amp? which bias current for the tubes?

Thank you, Daniele

Pinus wrote:
what do you think about that? Is it all original?

Other than the mains recepticle, it appears original to me. Your filter caps look to be in amazingly good shape for being 40 years old. One might be leaking (the 200uf, 2nd from the Power Transformer, near the power tube socket labeled "2"). Is there much hiss or hum while idling?

Your Hiwatt was not built with adjustable bias. So, your options to:

1) Locate and run only vintage Mullard EL34 tubes, as the amps were designed for them (note: this will be the most expensive option).

2) Purchase power tubes that are known to be tolerant of high plate voltages (the GT Mullard clones seem to do well in "hot" amps).

3) Install some adjustment so that you have more selection of power tubes. (see: http://hiwatt.org/DIY/bias.gif )

As I understand it, modern EL34's are happiest around 34-36ma.

Dave

Pinus wrote: I don't understand how to change the Power Tubes without set the bias, there are some kit already matched for this amp? which bias current for the tubes?

It is very easy to make the bias adjustable with one small 1-2 Meg potentiometer, as I did here:

viewtopic.php?f=2&t=1378

I can get the power tubes to idle around 42ma with this adjustment (as opposed to 60ma). No problems here. Everything sounds great.

Other than the mains recepticle, it appears original to me. Your filter caps look to be in amazingly good shape for being 40 years old. One might be leaking (the 200uf, 2nd from the Power Transformer, near the power tube socket labeled "2"). Is there much hiss or hum while idling?

Your Hiwatt was not built with adjustable bias. So, your options to:

1) Locate and run only vintage Mullard EL34 tubes, as the amps were designed for them (note: this will be the most expensive option).

2) Purchase power tubes that are known to be tolerant of high plate voltages (the GT Mullard clones seem to do well in "hot" amps).

3) Install some adjustment so that you have more selection of power tubes. (see: http://hiwatt.org/DIY/bias.gif )

As I understand it, modern EL34's are happiest around 34-36ma.

Thank for your answer, there aren't particular hiss or hum that i can hear, fortunatelly

The Nos Mukllard are really expensive i could see!

these could run?

http://www.tubeampdoctor.com/images/Fil ... .07.11.pdf

Which bias current with the NOS Mullard?

Pinus wrote: Thank for your answer, there aren't particular hiss or hum that i can hear, fortunatelly

Good! Visually inspect those filter caps on occasion, to make sure their bulges aren't growing and/or leaking.

Pinus wrote:The Nos Mukllard are really expensive i could see!

Yes. While original Mullards are the best valves to use in a vintage Hiwatt, their current prices have been driven beyond reach by audiophiles .

Pinus wrote:these could run?

http://www.tubeampdoctor.com/images/Fil ... .07.11.pdf

I do not have experience with TAD-branded tubes. If, indeed, that model can handle 500v, it should survive a Hiwatt. If you try them, please let us know your results. Otherwise, see if any of your more local vendors offer these:

http://www.groovetubes.com/iframeGT-EL34MQuartet.html

Pinus wrote: Which bias current with the NOS Mullard?

Original Hylight Electronics Hiwatts were designed with a fixed bias, intending to use Mullard EL34 power valves. Mullard's production was consistent enough to allow for tube changes without bias adjustment - and, wall current was lower back then. I haven't had a Hiwatt read as high as Ted B. has, but have seen 46ma before adjusting. That will melt many modern production tubes.

Dave

Another thing that helps is to ask the tube seller to send a set that is selected for low idle current. It won't fix the issue, but it will ensure the tubes can be biased comfortably.

In my particular situation, I can bias the tubes to around 34ma, but I have found it sounds audibly better when biased a little hotter (e.g. 42ma). I haven't checked the bias with Mullards, but I suspect they will also bias on the hot side without adjustments. I suspect Reeves intended it to be this way.

thank you for the help! Have you ever experienced with Black Sable SED EL34 ?

The regular SED EL34s are good. As for the Black Sable versions, they reduce the high frequencies, and many do not like them as much as the regular SD EL34s. I would not buy them - especially not at that price.

What's the difference between Ted B bias mod and OldSchoolDave bias mod? Have the same result?

I haven't tried Old School Dave's proposed modification. It appears as though one can use a small trim pot, but two additional resistors are required. I don't know anything about its range of adjustment.

The procedure I described is simple, easily reversible, stable, and effective. This is the same methodology we use to install adjustable bias on early Marshall amps. It provides virtually the full bias voltage available from the transformer.

Pinus wrote:What's the difference between Ted B bias mod and OldSchoolDave bias mod? Have the same result?

For the record, that's Mark Huss's bias mod, not mine .

As Ted B. alluded to, much depends on how much adjustment your amp requires. A good start would be measuring your current bias. That would reveal how much is needed to bring it into range.

FYI, one can also simply change out a resistor or two, provided that adjustment falls into common values. Long term, the adjustable trim pot is the better solution.

Dave

I have measured the Voltage from pin3 of El34 and ground are 420V, from pin 5 and grond -35.2 in stand by mode, and from pin3 to the central connector of the primary iof the Output transformer is 1.3V on a pair and 1.8V on the other pair. After i swapped out the valve with four matched JJ that i have on another amp and the value the first two value are the same, and the pin3 to the central connector is 1.6/1.7 V on all the valves. What this could be mean? How can i measure the bias current?

With the amp off and the power tubes unmounted and stanby switch in play position i have measured the impedance from the positive of the second filter cap from the right ([url]in this layout http://hiwatt.org/Layouts/DR103hiwattlayout.pdf[\url]) and pin3 of one valve and tester show me a value of 22.5 ohm.
So 1.6v/22.5 = 71,1 mA is the bias current? or is 71.1 / 2 ?

Once the amp is off and the caps are fully discharged, measure the resistance from the center tap of the OT to pin 3 of tube one and four. It should be around 22ohms. Divide the voltage you measured across that same circuit by this resistance, and you'll have the current flowing in each half of the secondary. Divide that by two to calculate the approximate current flowing in each tube. There is no easy way to measure the individual currents since two tubes on each side are connected in parallel.

--mark

Thank you

So i measured
plate volt. = 420V
plate current for each tube= 36/37 mA
grid negative voltage = -35.2

the bias current would be 0,7 x25 /420 = 41 mA or it's good as i have measured?

Top values obtained with voltage selector in 245V, here we are we have 230v at the walls.
I repetead it with voltage selector in 225v position, and the value seem to be more goof:

Plat Voltage : 460v
Grid bias Voltage: -38v
bias current for each tube: 41.5 mA

Pinus wrote: Plat Voltage : 460v
Grid bias Voltage: -38v
bias current for each tube: 41.5 mA

This appears to be correct.

Enjoy.