Now that’s a Proper Meter

H/P 400 LR. AC VTVM

Could be turned into a mic pre but I’m using it for alighment

Here is the under side.

http://www.mcmlv.org/Archive/TestEquipment/HP400.pdf
Sine wave In has crossover distortion at output binding post . This thing is old so caps are First to replace.  The measured voltage is very close to reading
V5 tube’s cathode drives the output .  It could be a tube pre with a ef 86

It would be a Sin to cut that up. Especially an "L"! It is genuinely useful on any (large) audio workbench.

It is a Ballantine without the smell of Boonton.

>  crossover distortion at output

The external output will always glitch at zero-cross. NFB is wrapped around the meter's diode bridge, which non-conducts through zero, throwing the NFB in a tizzy. Short all 3 diode-nodes together when a clean output is wanted.

Hiss level may be quite bad. 6CB6 is not hissy but ALL the gain is always wide-open, loss is taken before any gain. I suspect few studio uses will work with less than 20dB loss ("-40dB, 0.01V") which makes noise figure at least 25dB where we like less than 6dB.

Thanks PRR for explanations of crossover distortion.  I thought it had something to do with diodes in my circuit also the output is a bit noise but handy to drive a scope while checking clipping. 

My brother is a believer in small work bench with large amounts of storage.  The HP is great for working on analog but Large as you say.  I’m trying  to reduce desk space but can’t get rid of gear .  I need an intervention.

I recently had a Waveforms 520A restored. It is a very compact AC VTVM. I love that thing. It’s the only piece of tube gear I use including the studio. When I helped clean out a large old shop the Waveforms Transmisson Line  units went in the dumpster. No takers. They were the size of that HP but had an oscillator and a meter. Very nice units but too large for your average bench.

Hi All
PRR’s explanation of the crossover is correct. The meters rectifiers add distortion to the normal output signal. I have modified my HP 400FL and others to make the “External Monitor Output” clean. I have added an opamp (NE5534A) with a gain of 20dB. The signal is tapped before the Meter Amplifier (after the range switch) and the signal is very clean. I hope this may be useful to some of you.
Duke

The output buffer is a good idea.  I wish  I had an FL due to size but I’m thinking. Of how to modify my bench to weave into the top at. An angle so as not to loose much space.  I would have to come up with a Pwr supply for the opamp. Since my. Pwr supply is for Tubes.  FL would be much easier use the exsisiting supply.

Audio1Man said:

Hi All
PRR’s explanation of the crossover is correct. The meters rectifiers add distortion to the normal output signal. I have modified my HP 400FL and others to make the “External Monitor Output” clean. I have added an opamp (NE5534A) with a gain of 20dB. The signal is tapped before the Meter Amplifier (after the range switch) and the signal is very clean. I hope this may be useful to some of you.
Duke

Click to expand...

Back when, I looked at the product output from my (very) old test bench, that was put together on a (very) tight budget. Kids today do not appreciate how expensive good bench instrumentation was.  I bought an old spectrum analyzer (something like $700 for an old used Singer SA). Not much to speak of by todays standards, maybe 50 dB total dynamic range and 30 kHz bandwidth... but good for my design bench almost 40 years ago. Since the SA by itself was marginal for fully quantifying circuit performance, I fed the product output from my (cheap) Heathkit distortion analyzer into the SA input. The product output of the distortion analyzer is a deep notch filter with gain... feeding that output into the SA input extended my bench measurement floor a few more tens of dB.

As I recall if I operated the distortion analyzer at nominal 0dB, I could see the distortion caused by the Heathkit audio path.  Lining up the Heathkit distortion analyzer 10 dB cooler @0=-10dB, dropped the Heathkit's added distortion below it's noise floor, while still giving me decent total measurement resolution.  8)

Nowadays you can just about do this with a smart phone ap... and my Rube Goldberg test bench is ancient history, but back in the day it helped me measure stuff I could never hear, to improve my designs. 

JR

Duke - this is a great idea - the distortion in the monitoring section of my HP400FL and HP400GL has always been a real bummer and driven me nuts - Im very interested to try this out - do you have any sort of drawings to detail this?

are you tapping signal to the meter from C38/CR22 and C39/CR23 just as the original meter is drawn?

what about power?

thanks Duke,
Timothy

Audio1Man said:

Hi All
PRR’s explanation of the crossover is correct. The meters rectifiers add distortion to the normal output signal. I have modified my HP 400FL and others to make the “External Monitor Output” clean. I have added an opamp (NE5534A) with a gain of 20dB. The signal is tapped before the Meter Amplifier (after the range switch) and the signal is very clean. I hope this may be useful to some of you.
Duke

Click to expand...

Does anyone using a  solid state 400 with DC inputs use a battery?  What batteries do you use?  Having the meter float would be useful sometimes. 

Hi Timothy
Connect the OPAMP to the main signal from the attenuator switch to base of Q10. This signal is clean. Any connection after this  will have METER RECTIFIERS DISTORTION.

The opamp is AC coupled input and gain of 20 dB I added 10 volt zeners and decoupler caps to the supply.

I also added a switch and additional BIGGER CAPACITOR to change the BW from 100 kHz to 400 Hz for HUM MEASUREMENTS.

I don't recall the specifics as it was done 15-20 years ago. I hope this helps.
Duke

> solid state 400 with DC inputs use a battery?

TWO >35V batts at 45mA.

More than a stack of 9V pedal batts can do.

At this level I would consider a motorcycle battery and one of those cigar-lighter 120V inverters.

PRR said:

> solid state 400 with DC inputs use a battery?

TWO >35V batts at 45mA.

More than a stack of 9V pedal batts can do.

At this level I would consider a motorcycle battery and one of those cigar-lighter 120V inverters.

Click to expand...

If there is such a thing as a 35v or more battery I haven’t found it. It looks like you have to stack 12V batteries. For two meters that’s a dozen batteries. For 36V.

36V is an unusual voltage for a battery charger so it’s probably a pair of 48V strings plus a solar style charger. That’s expensive. I was hoping there was a less expensive way.

Use to be dry cell 45 and 90 v batteries for b+ pwr for radios.  I used 2 90v batteries to bias the reeds on my Wurlitzer. They kind of disappeared with the old electronic part stores.  I guess use some battery packs like 2. 20 v recharge tool battery’s

Gold said:

If there is such a thing as a 35v or more battery I haven’t found it.

Click to expand...

https://www.lithiumpros.com/product/lithium-pros-m3140-36-36v40ah-deep-cycle-lithium-ion-battery/


$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

At $2199.00 that’s  close enough to not existing for me.  The meter doesn’t draw much current. If I could find a low capacity low price 12v battery I might go for it. Otherwise it will be unbalanced operation only.

Yeah I thought it was funny...Pretty cool though....

Those little mini atv batteries are 12v I believe......

2 Dewalt 20 volt batteries would be what ,  Maybe $50-79 apiece.