HT Stage 60 Valve & Speaker upgrade

[Maxsys]

Hi al,

so i finally got round to upgrading my speakers and valves, all i can say is WOW, what an improvement.

Ordered from https://www.watfordvalves.com/

2 x Classsic Ruby 50H speakers

Diameter-12 inch
Power Handling- 50 watts RMS
Nominal impedance-16 ohms
Chassis style –British Vintage Style 4 hole
Chassis colour- Ruby Red
Chassis material-Pressed Steel
Sensitivity -99.5 db
Magnet-Ferrite
Magnet-6 inch 50 oz
Voice coil-1.75 inch
Frequency response 75-5k
Resonance frequency- 75 HZ
Weight UN boxed- 4.85 kg

Classic Cryo Revalve kit
The kit consists of
2 Specially Selected Harma ECC83 Retro Cryo
1 Specially Selected Radio Technique ECC82/6189W Cryo
2 Specially Selected and matched Harma Retro EL34 Cryo

All in all very pleased, biased the power valves @70mV for the pair

[thephantum]

Very nice. I'll bet it sounds awesome. Enjoy it.

[Maxsys]

Yep sounds great, the original valves were running quite cold, about 54mV when i checked, thought i would check before i replaced them. Put the new ones in powered up and they showed about 65mV so slight teak to 68 and all is good.

The speakers are awesome, the fizz and icepick top end is now gone, nice smooth OD sound and superb cleans.

Very pleased and the service from Watford is second to none, you can see why they supply some of the most famous names in the industry.

[Dynochrome]

Do you recall what the plate voltage was? That seems high for a EL34 amp.
Guessing 380 PV 25W / 380 = .066 x .7 = .046

[Maxsys]

Hi there,

yes the plate voltage was around the 500 mark, cannot remember exactly, i biased the amp checking the plate voltage and using the two resistors.

[thephantum]

Plate voltage on the HT-60 typically lives somewhere between 485v and 495v, which is the upper end for a EL34 amp.

[Maxsys]

OK now i am concerned i have done this wrong or am reading it wrong.

Just took the amp apart again, see images, measuring on each pin 3 i am getting 529v on each both the same. Positive lead to pin 3 negative lead to chassis.

Measuring at resistors i am getting 68.3v neg & pos as shown in the image.

Am i doing something wrong?

Amp is on, speaker plugged in, cable plugged into input, all dials set to zero

[Maxsys]

OK now i am concerned i have done this wrong or am reading it wrong.

Just took the amp apart again, see images, measuring on each pin 3 i am getting 529v on each both the same. Positive lead to pin 3 negative lead to chassis.

Measuring at resistors i am getting 68.3v neg & pos as shown in the image.

Am i doing something wrong?

Amp is on, speaker plugged in, cable plugged into input, all dials set to zero

[Maxsys]

Right quick update, i put the old valves back in, took the measurements again from the same points as shown, each plate was 539v across the resistors 50mv

so question is what is wrong if anything, am i doing this right or not?

[thephantum]

You are measuring plate correctly. After doing the math, it's actually right on the money. EL34's are rated for a maximum plate voltage of 800V. At that level, a pair of them in a Class AB Push Pull config will put out 90 watts.

So in a 50w amp you would see around 445V or so. For higher wattage's, most manufacturers move to two push pull pairs (4 tubes) to make power...so you usually don't see plate voltages over around 450V in EL34 amps. However, this is a 60w amp using one push pull pair. Doing the math, yours is living right where it's supposed to...you said it was 529V now and 539V with the old tubes. The math says it should be 533V.

I'd be interested to see what you would measure using a probe...I'll bet it will read a slightly lower voltage (closer to 500V) since it's reading the voltage drop between the Plate and Cathode...not between the Plate and ground.

You are biased a little warm with 68.3ma @ 529 Plate Voltage, but only a little...72.3% of max plate dissipation vs 70%. Nothing wrong with that...it's certainly not going to hurt anything and it probably sounds pretty awesome.