modulator tubes in apache

[KB5MD]

Does anyone know of a suitable substitute for the EL34/6ca7 modulator
tubes in a Heath Apache TX-1? Or, has anyone modified an Apache to use some other modulator tubes?

[W3SLK]

The 6CA7 is a fine tube in and of itself. But if you want to go out and mod it, maybe a pair of 6550's. But I would stay with the EL-34/6CA7's.

[Todd, KA1KAQ]

I'd second what Mike(y) said, stick with the EL-34s. Unfortunately the audio crowd is willing to spend big $ for these as well as the 6550, so it's not really worth the effort as long as the tubes are readily available IMO. You'll find that many radio folks are willing to sell you audiophool tubes very reasonably if they know you'll be using them in your transmitter and not flipping them on ebay.

[WA1GFZ]

811A

[W3FJJ]

Well, don't do what I did. When I was a johnny Novice, I ripped out the EL34's and put
a pair 6146's. Reason being, I had schematic for a modualtor using them,6146 AB1 with Phase splitter.
I never did finish the prject , But still got rig, so maybe someday.

ANyway looking back, there's nothing wrong with EL34's, so best to stick with them,
I see you can buy brand new  Svelenta russian EL34 for  $15.95, I guess they are ok..

[WD5JKO]

I vote for the Penta Labs E34L which is a beefier replacement for the EL-34:

http://www.pentalabs.com/pdfs/EL34S.pdf

I bought mine from Penta labs directly and the price wasn't too bad.

Jim
WD5JKO

[N2DTS]

I have used KT90 tubes in my 32v3, good hifi tubes, run AB1 with no driving power, and are good for lots of power.
I got some Eastern European ones cheap, not Russian...

They make very good subsitutes for 6146, 807's, 1625,s and so on.
The KT90's are rated for 850v on the plates I think, there are also KT100's...

Brett
N2DTS

[John K5PRO]

Hmm, I spent time and effort testing 6CA7/EL34, 6146 and 1625 (807) original tube in my DX100, and found that the 6CA7 offered the best peformance, both audio quality and modulation drive for given HV and screen V that I could coax out of the -100. I wouldn't switch, my DX100 is now EL34 modulated. The 6550 or other heavier tubes will required at minimum a new filament transformer as well as needing a lot of space to fit the fat bottles. Won't work unless you build an outrigger modulator!

[N2DTS]

I think its a good idea to remove the vacuum rectifiers and solid state the HV, that allows you to offload the filiment power, reduce temps, and you get sockets for VR tubes to regulate the screens of the mod tubes.
I never found the extra hv to cause problems unless something else is wrong with the rig, which should be fixed anyway.
If you VR tube the HV, you can remove the tap off the hv supply (between the filter caps) and maybe that wont unbalance the caps and contribute to failure, you might even be able to run a nice oil filled cap in place of the marginal electrolytics....

I like AB1, you can ditch the postage stamp sized driver transformer!

Brett
N2DTS

[flintstone mop]

I am not talking from experience, but I never thuought that a 6146 was a very good audio tube. It seemed happier in the RF service. Anyone here find 6146's to be great audio tubes?
Is the secret the way it is biassed?? Screen volts?? Or the odd plate impedance and what is available for mod transformers?

As for 6550's I can hear a nice difference in the transmit audio on my Elmac AF67 and sticking the 6L6's back in. Huge bottles but they fit in there and I attached a little coolling fan that runs during transmit. Got 700 vdc on the plates. A lot of audio!!

fred

[Tim WA1HnyLR]

my self, I would stick with the EL34 tubes or go to a pair of big tv sweepies like 6DQ5 tubes with 150 VDC regulaed on the screen. The 6550 in theory looks good but the 800 VDC plate voltage could be problamatical.6146s are poor audio tubes unless they are idled at 80-90 ma on a pair. At 800 volts you will cook them. We must ask ourselves, why do 6CA7s crapout in the modulator of the Apache or flash over at the socket? This is caused by the deliberate mismatch of the modulation transformer design. The tubes are undercoupled to the class C load in order to cause high level clipping as one approaches 100% modulation. Very high peak voltages build up as a result of this operation  and cause breakdown. There is a cure . It is known as turbo connecting or autotransformer connecting the modulation transformer. This is done simply by disconnecting the bottom of the secondary from the B+ source. Connect the 500 ohm tap to the appropriate modulator tube plate to create an in phase condition within the transformer. To figure this hookup out do the following. Connect an AC voltage across the primary of transformer .I typically use a suicide cord and connect 120VAC directly to the primary. Make sure that the transmitter is on a work bench and the case NOT grounded as there will be a short to ground through the power supply filtercaps to ground. Chassis will have 60VAC on it with respect to ground. You can disconnect the centertap of the primary for this test if you would like. Connect the 500 ohm tap to one side of the primary with a clip lead. Measure with voltmeter from primary centertap to top side of secondary( modulated B+ to PA stage) you should see about 95-100Vac. If the figure is low then connect clip lead to other side of primary. Incorrect hookup will zorch out the modulation transformer. After determination of correct hookup ,remove clip lead and make permanent electrical connection. This configuration will give you an impedance step down of 1.5:1 This will allow you to make 125% positive peak or better. The modulator tubes will be more reliable . Being that a greater portion of the transformers windings will have the unbalanced DC load presented by the 6146 PA tubes.You can't load to PA plate up to 250 MA like once done. the mod transformer will satuatew at the old plate current value 175 Ma is the figure now. You will still get about 100 watts out of the transmitter with a solid stated HV supply. The reduction in power is inconsequential. An additional 10 K 10 watt resistor must be added in series with the PA screen dropping resistor to bring the total value to 30K.the original value of 20K is too low and will cause overdissipation of the 6146 screens.An additional resitor of 4.7-6.8k 2 watts should be wired in series with the grid bias feed to the 6146 grid circuit for appropriate additional grid leak bias. Power supplies MUST be solid stated. The HV B+ will be in the order of 800V or slightly more. The low voltage supply MUST be changed to choke inpoot simply by removing the input filter capacitor from the rectifier side of the filter and paralleling the cap with the outpoot filter cap. This results in a power supply voltage of 270 volts or so. The 6CA7 tubes are much happier with that voltage on the screen. Assuming that this work is done in it's entirety do not leave out any steps you should be very pleased with the results. De Tim WA1HnyLR

[flintstone mop]

Thanks Mr Tron,
That's where my beautiful TX audio comes from for the Elmac. I performed the old TV or old stereo Tube amplifier power transformer trick.
yessindeedeee lottsa modulation now.

The "turbo" mod for the Apache tranny makes a big difference.
Fred