Replace EL34 modulator tubes in Apache

[w5gw]

Am contemplating replacing the EL34s in my Apache with 6146s - has anybody done this.

73

Gary W5GW

[W3RSW]

Why?

The EL34 is already a nice, low distortion match for the voltages present.

[flintstone mop]

Ouch!!
The 6146 is not a great audio tube.

[WA5VGO]

This shouldn't be too difficult and should work well. I see two issues. Heathkit is running about 350 volts on the screen grids of the EL-34's. This is way too much for 6146's. You need to reduce this to around 200 volts. Three 50 volt 5 watt Zener diodes is series would accomplish this and wouldn't upset the regulation. The other issue is bias. It looks like the EL-34's run at about -35 volts. The 6146's will need about -50 or so. If the bias adjustment has this much range in it you're okay. If not, you'll need to juggle some resistor values around to achieve it. 

Darrell

[N2DTS]

I have heard people using 6146's as modulators that sounded good, but I was never able to get them to sound good.
They make kt88, kt90, and kt100's now, I have a set of kt90's in my 32v3 that sound great.
The hifi tubes seem to want higher screen voltage.
Way back in the dx100 days I solid stated the vacuum rectifiers and installed the big VR tubes in their place to regulate the screen voltage.
I use three smaller VR tubes in the 32v to get 450 volts.

[Opcom]

I'd use octal based sweep tubes before using 6146s. They also use a lower G2 voltage, and can conduct full current with a low G2 voltage. Maybe get more plate voltage swing or less abuse of the screens than with the HV-loving screens on EL34s. Octal sweep tubes also stand lots of plate volts and are cheap. What is the issue with EL34s? I use the $12 China ones with no trouble.

you got 1.5A heater current per tube to play with.
6BG6 basically a 6L6  fil.=0.9A
6CD6 and 6DQ5 are tough b*****ds but need 2.5A
6DQ6 needs 1.2A

[John K5PRO]

6146 also needs more area around the socket to accommodate the wide body. EL34 is a beautiful tube for audio service, and much improved over the 1625s used in the DX100. Johnson did make good use of a pair of 6146 with a trio of the same in the big Valiant. I swapped a DX100 to EL34 and built a small solid state regulator that picks off the screen voltage at 300.

[wa3dsp]

I updated a Ranger (not Ranger II) a few years ago and I replaced the metal 6L6's (actually 1614's) which were stock in the Ranger with EL34's.  They seemed to match the size and higher voltages due to line voltage increases that I had to work with. I had considered a 6550 or a KT series but both were physically larger and a tight fit with the metal shield and generally overkill modulating a single 6146.

I will admit I had the EL34's and I am not sure what I would have done if I had to buy them. All audio tubes are sky high in price. For that reason along a 6146 might be a good choice. Most hams have an abundance of them lying around. Also the high voltage tends to run high so 650-700 volts is not uncommon and well within specs for the 6146. Since you asked about a 6146 I use that as an example. There may be better choices.

If you have good EL34's I would leave them in.

http://www.crompton.com/hamradio/JohnsonRanger/RangerRestoration.html

[Jim/WA2MER]

I will admit I had the EL34's and I am not sure what I would have done if I had to buy them. All audio tubes are sky high in price.

Not sure what you mean by "sky high," but I'm running current manufacture EL34 tubes (Electro Harmonix) in my Apache that I bought early this year for $29.98 a pair, shipped. So far they haven't melted down and no one has yet complained that I made their ears bleed. There are quite a few Asian and Eastern European manufacturers (mine are from Europe) of audio tubes. Now if you're convinced that your Apache needs NOS MullardSol black plate (no holes) metal base tubes with the "O" getter and oxygen-free glass envelope that was evacuated in a zero-gravity environment, you will pay dearly.

[VE3AJM]

An arcing problem at the octal tube sockets or tube pins on the 6CA7/EL34 modulators can develop between the plate and screen grid connections in the Apache tx.

That may give you reason to use different modulator tubes, ie. a sweep tube such as a 6CD6 or 6DQ5 etc or a 6146 which use plate caps, thereby removing the B+ from the octal tube socket after having cleaned the octal sockets and or replaced them if there was any arcing.

Al VE3AJM

[pa0ast]

Most of the EL34 sold on this moment are junk.
You can use them as long as the anodevoltage is lower then 400 Volts.
If You go higher in voltage then there is just one brand that is really good on this moment up to 750 Volts.
That brand is JJ , new production made in tjechoslowakia.
Ask the guitar amp boys. Only the  JJ brand  is OK for new tubes they say and swear by it..
Prizes for nos , philips, valvo, telefunken where high but dropped due to the good tubes of JJ.
My amp does 140 watts sine wave out of 4 EL34's with philips or JJ tubes and a long life span.
more info : the internet.
http://www.jj-electronic.com/index.php?option=com_content&task=view&id=20&Itemid=47
http://www.thetubestore.com/Tubes/JJ-Electronics
RGDS Anton

[WBear2GCR]

What really needs to be considered is the plate Z for a given tube at the operating conditions. The Apache (and any other) audio transformer is going to give the max output power when properly matched.

I don't recall exactly now, but one can compare the DX-100 mod iron with the Apache. The difference is in the turns ratio, and of course the tubes used.

A good example are the Dynaco MkII and MkIII. They are identical amplifiers except for the output transformers and tubes. One uses 6L6/6550 and the other EL34/6CA7. Switch tubes and neither amp will make their rated output. The match is not right.

Otoh, within reason a tube with excess current may make up for a Z mismatch, if it can be driven sufficiently hard. But the drive is limited in the stock form of the Apache, iirc.

Either way, the 6146 is not favored by anyone as an audio tube.
Perhaps a set of 4D32 would be interesting (assuming they can fit).

                                     _-_-

[N2DTS]

I think a 4D32 is just like two 6146's in one bottle, and not a hi fi tube.

A few years ago, my wife got me four Ei Magnoval KT90 tubes for $90.00.
They sound better then the real KT88's in my 32v, and are not as wide.
I think they are made in Yugoslavia.

If you can find tubes that you need made by them, you should have good luck.

[WD5JKO]

An arcing problem at the octal tube sockets or tube pins on the 6CA7/EL34 modulators can develop between the plate and screen grid connections in the Apache tx.
Al VE3AJM

  I second what Al says. My first modulator I built was a pair of 8417's mounted in ceramic Johnson sockets. What could go wrong? Well the plate is pin 3, filament pin 2, and screen was pin 4. In my case instead of arcing from 3 to 2, or 3 to 4, it arced pin 3 to chassis. That stopped after I lowered the socket using nylon washers. Then there was the occasional spark from 3 to 2...

  There are sweep tubes without a plate cap that escape the issue by moving the plate to pin 5 and then omitting pins at 4 and 6. The 6AV5 comes to mind here, but a pair might be a little wimpy for a Apache...that is unless one drives them with screen grid drive. The link below gets over 80 watts RMS out of a pair with only 550 volts on the plate. A quad of 6AV5's run conservatively should do a good job when using screen grid drive.

http://tubelab.com/articles/tube-testing/6av5-sweep-tube/

Scroll down about 3/4 of the way when he switches from single ended to push pull..

As to the best EL-34, my vote is this one: (E34LS by Teslovak)

http://www.pentalabs.com/Product.aspx?dept_id=1516&pf_id=e34ls%20mp

It has a beefy plate like the tri metal 6L6GC of years past. I ran one as an RF amplifier with 750V on the plate. Several times overloaded by accident which the tube withstood. I tried some other brands EL-34's (not JJ or EH), and a single overload would go poof! The B+ gets shorted out from a gassy tube.

I think it's time for the guy who started this thread to better describe why he wants to switch from EL-34 to 6146. He might have a good reason.

Jim
Wd5JKO

[Opcom]

An arcing problem at the octal tube sockets or tube pins on the 6CA7/EL34 modulators can develop between the plate and screen grid connections in the Apache tx.

Al VE3AJM

A friend who in the days of the USSR's Eastern European dominance built many bootleg copies of Marshall and other guitar amps running 800V on EL34's found that ceramic sockets solved his arcing problem.

Either way, the 6146 is not favored by anyone as an audio tube.
Perhaps a set of 4D32 would be interesting (assuming they can fit).

Two pairs of 6146s in AB2 always sounded great in my old Chevy van. Best use of 14V@30A ever.

[N2DTS]

I have/had some ceramic sockets that had the pins recessed in the socket with walls between each pin.
High voltage versions of an octal socket.
They may have come out of an old audio amp...

[VE3AJM]

Hopefully Gary will chime back in and let us know why the possible use of the 6146s in his Apache.

It wouldn't be my first choice as an audio tube either, but I was considering the little brother of the 6146. The 2E26 tube.
A pair are used as a PP audio driver in the RCA BTA 1R1 broadcast tx here. There is no driver transformer used and there are negative feedback ladders around the 4-400 modulators in AB1, back to the secondary of the balanced input stage. So with a sensible thoughtful design, they work very well in this application as an audio tube.

Al VE3AJM

[wd8das]

Back in the 1990s I bought an Apache TX-1 cheap that was missing the modulator tubes and all the low-level audio stages were stripped out.  That suited me as I was intending to re-do the audio entirely.

I converted it to use 6146's as the modulators because I have plenty of them on hand.  I drove their grids with a receiver push-pull output transformer.  This gave me a low-Z input on the rear apron that I could drive with a small utility amp, in turn fed by a DBX compressor/limiter with a D-104 mic into the instrument jack. 

It all worked like a charm and sounded great!  It is still my best-sounding *amateur* transmitter.  The only thing that beats my Apache for quality are my two Gates BC-1T converted broadcast transmitters.

I know it was a long time ago as the schematic was drawn in pencil on graphing paper.  Here's a scan of the drawing with some added details:

http://www.wd8das.net/TX1MOD.jpg

Steve WD8DAS

[w5gw]

To answer why:

One of the reasons, arcing of the EL34s, next is cost. Attempts to eliminate arcing has been met with marginal success even tho I have put in new ceramic sockets.  I've had marginal luck with Chinese EL34s. Finally, while I usually keep my vintage gear stock, I'll likely rip out the audio driver anyway. I would put 807s in there but they won't fit and I have plenty of 6146s.

Thanks for all of the comments and suggestions.

73

Gary W5GW

[N2DTS]

Gary,
There were 807's made that are shaped like 6146's, wide base, short and squat, I think they were used in some military radio.
They would be just slightly taller then 6146's.
I have some  and will measure them tonight.
I have no use for them, so if they work size wise you can have them.

[N2DTS]

6146, 807w and an 807.

The 807w is about as wide at the base as a 6146 but is not as tall.
807's were the tube of choice for 100 watt modulators.
The B+W 5100 used 6146's I think, just to be different?




http://n2dts.smugmug.com/Ham-radio/i-ZvFP5rj/X2

[wa3dsp]

I don't get this arcing problem. What is arcing? The tube or something external? Are you in a high humidity environment? 6146's are run in many transmitters at 800-900 volts with phenolic sockets and no problems. Certainly ceramic would be better and preferred. I run 7027A's at 650-700 volts in the Ranger without problems. The tubes I used were NOS American made so maybe the current foreign ones are giving problems.

I actually miss spoke in my earlier post when I stated I used EL34/6CA7. I used 7027A's in my Ranger update. That is the tube that is used in the Ranger II modulator. I am surprised no one called me on that! Anyhow the 7027A's run fine at 700 Volts on the plate. The screens and all the low voltage in the rig are regulated at 300 Volts.   

[N2DTS]

He said the tube sockets arc, because the plate comes out the socket.
I run kt90's in the 32v with the plate out the socket at 700 volts without any problems.

[VE3AJM]

Tim WA1HLR opined about the cause and solution of the arcing back in 2009. Makes a good case for adding spark gaps to the primary of the mod transformer too. Amazing what you can find in the archives on here :

Al VE3AJM

Title: Re: modulator tubes in apache
 Post by: Tim WA1HnyLR on January 29, 2009, 01:34:46 PM
--------------------------------------------------------------------------------

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

[ve3bkd]

HI , HERE ITS SOME PICTURES TO SEE OF A MOD DONE BY MY GOOD FRIEND , VA3-POS , ON MY APACHE WITH HAMOND AUDIO TRANSFORMER ,AND TURBO MOD ,ETC ,AS TIM TROM SUBJESTED ,BUT WITH HIS OWN CHANGES.HOPE IT CAN HELP.