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jhstewart9 7th September 2019 06:16 PM

Info for fans of the 50
2 Attachment(s)
Some History. The 50 was quickly replaced by the 45.

jhstewart9 8th September 2019 04:44 PM


Originally Posted by disco (
After exchanging my easy playing 4 ohm speaker for a nasty 8 ohm bass reflex (with lots of caps and coils going on) reproduction suffered from boom. The lowest register was somewhat out of control. A simple exchange of the 27 and 50 cathode bypass (to HT) made an improvement. Not belonging to the incrowd familiar with Chebyshev's theorem I took the empirical route. Output impedance must have dropped just enough to make a difference. Exchanging this lil tubegiant for the old Marantz PM7200 made no difference in bass reproduction.

Damping looks on the low side at <2. That since rp is ~1.8K driving an OPT set for 3.5K. Then wdg resistance of the OPT primary & secondary need to be plugged into the DF equation as well.
Many people design for the OPT to be ~3rp, resulting in better DF & lower D%.:)

disco 9th September 2019 10:56 AM

Yeah, I was thinking along that line too. But when I connected the 8Ω speaker to the 4Ω output (offering a higher impedance transformation ratio) there was not much change. It seems the amp is running out of steam at high volume, as most small amps do. One or other speaker makes a difference I noticed. Mind you, differences are marginal but compared to solid state this triode amp has better grip on low notes with the first speaker I tried than with this second. More speakers should be tried before coming to any conclusion.

It passed my mind the opt could be the cause as it's a large 40W GOSS unit, allowing for 130mA. Perhaps the #50 quiescent current is too low for reaching its linear transfer charcteristic. Midrange and high frequencies are good though and primary self inductance should also be constant with quiescent current.

jhstewart9 9th September 2019 01:01 PM

1 Attachment(s)
If you have a scope & audio signal generator use them, look at the output & sweep the low frequencies. Poor DF will shew the speaker resonance(s) immediately. And you will hear all in a direct way, see where the problems are. A better test than your ears alone. Otherwise you are guessing & flying blind.:)
In your test setup I see many long leads. If they are in the signal path there is a good possibility that parasitic RF oscillations are occurring under some conditions. That will cause very poor audio. The setup looks dangerous as well, very high voltages running in long leads.:eek:If you don't have the test equipment find a friend who does.

disco 9th September 2019 03:17 PM

I did do some tests and this is what I got. Input is 0,7Vrms sinus from the signal generator (dropping 1 to 2% with falling frequency, not compensated), output in mVrms taken from the 4Ω tab. First column= frequency, second= 8Ω loudspeaker, third= 4Ω loudspeaker, fourth= 8Ω resistor.

Freq LS-8 LS-4 8res
10 0173 0144 0154
20 1122 1640 0985
30 2645 2795 2401
40 3704 3168 3077
50 4950 3822 3470
60 5990 4640 3710
70 5840 5140 3810
80 5330 5150 3860
90 4920 4920 3847

jhstewart9 9th September 2019 08:49 PM

1 Attachment(s)
That is good data to work with. I've plotted your measurement results onto a spread sheet, a resonance is clearly visible on the 8Z speaker. The plot & spreadsheet are not really needed but it better illustrates the operating conditions.

If it is in a bass reflex enclosure there may be another resonance just above 100 Hz. In that configuration there are usually two peaks. Are there any crossover networks in the speaker cabinets? Also should do a measurement at mid-band (400 or 1000 Hz) so that a comparison can be made with the LF, especially at the resonance. But looks like both speakers are not well damped. I'll have another look at your data tonight.:) 8R speakers commonly measure ~6.5R on an Ohm Meter.

jhstewart9 10th September 2019 02:00 AM

Rough Estimate of Damping Factor
First get a rough estimate of the amplifiers internal resistance as seen by the load. The audio generator should be set somewhere near the mid-band of the amplifier, 400 to 1000 Hz, not critical.
Be sure your DVM is still accurate at the frequency set, some older versions are made for power frequencies only on the AC ranges. The spec sheet will tell you.

For your 50 Amp drive, it with a signal that will produce about One volt, again not critical, on the output terminals connected to an 8R resistance load. Measure the resulting voltage. Then disconnect the load resister & measure again.

The internal resistance is simply the change of voltage divided by the change of current in the load resister. Ohms Law.:) By comparing that with the RATED (8R) load, the DF is 8R / Internal Resistance.

If the internal resistance calculated from your measurements was say 3.5R, the DF would be 8/3.5. A well designed triode amp would have a DF of maybe 3. That would assume an OPT of >3 rp. All triode amps without NFB depend very much on the OPT selected.

disco 10th September 2019 09:38 AM

It's a Fluke 87, good for 10KHz at least. Tomorrow I'll rerun tests with a 5K opt, see what it brings. Also, I'm reconsidering the input stage, replacing the Chebyshev filter with an active load. To be continued...

Freq LS8 LS4 8R
100 4180 4620 2823
105 4120 4500 2865
110 4070 4410 3188
115 4025 4430 3210
120 3985 4170 3258
125 4060 4100 3583
700 5010 3748 3615

0,7V @700Hz, no load = 5710mVrms

jhstewart9 10th September 2019 04:23 PM

1 Attachment(s)
Something wrong with the last set of numbers for the loaded & open circuit voltages. That looks like what a pentode would do.:eek: The attached article is very good, perhaps you have already seen it.:)

jhstewart9 10th September 2019 08:55 PM

2 Attachment(s)

Originally Posted by disco (
Extract from the RCA paper:

Setting the dominant LF roll off at the front end prevents things like turntable rumble & other such LF from getting into the amp. If they do there will be intermods on the program material. And large speaker cone excursions when we don't want them. On the pair of 6L6GC amps I built around 1960 I stuffed in a 2-pole LF filters on the front end.:)

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