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 Post subject: Re: 10DE7 tube
PostPosted: Nov Wed 08, 2017 8:08 pm 
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Very interesting.

Is it any better audio quality wise than the modulation used in the 6GY6 or 6888 transmitters?

This could be an easy cheap way to get a push pull triode amp.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Wed 08, 2017 11:34 pm 
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May be a illusion but audio does have a edge... BUT it's fed from a different computer than my normal 9KC6 xmitt, this one has a graphic EQ in the mix(yeah I should just turn it off but it's out back, plus it's raining & cold)... Using my TV tube radio, both bass & treble controls have a noticable increase in their function, as does tone on my 4586 Silvertone console...

I'll perform a comparison of both Tx, but need to make some rearrangements to use the sweep generator... With the freq generator for osc drive, HV power supply, modified 6888 Tx & 6DE7 amp, bench is getting crowded... I can't can't cleanly modulate it using the onboard VFO in the 6888 Tx, need to devise a method to reduce it's output... One thing for sure, the dual control pentodes are far more forgiving of slight misadjustments...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 09, 2017 12:54 am 
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Cool.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 09, 2017 3:34 am 
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I'm on the edge of my seat.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 09, 2017 5:15 am 
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LOL, OK going to be a couple days... I have grandkid duty at daughters house from tomorrow at noon till approx same time fri... When I get back home I'll have to troubleshoot it before doing any testing... Earlier was kicking along just fine and lost carrier back here to house, inside garage it was still making weak & distorted signal... Something is intermittent as it had a couple quick dropouts earlier... I'm going to shotgun it and replace the RF related components in output as many of those have been disconnected numerous times in previous versions, suspect a broken connection inside part...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Sun 12, 2017 5:11 pm 
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It's fixed, was a cold solder connection on the switch I used to install Bob Weaver's antenna matching toroid...

In other words operator error... :oops:

As far as frequency improvement, I haven't made a determination... The transformers I have on hand won't produce enough voltage swing to give anywhere nearly full modulation... Puts freq tests on hold till I come up with a transformer(or at least a circuit) that can produce higher modulation levels...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Tue 14, 2017 4:13 am 
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He, he, he, it's a workin'... Image

No I haven't checked frequency response, mainly pulled out the 6AQ5 that got mixed in with the 6BF5 I started with(three NOS & two used)... Yep, again operator error...

I've found reason 6BF5 work so well is they'll drag down B+ to around 90v, so my transformer will modulate the thing... Before going farther I'm going to reduce B+ to see if a 6AQ5 will give similar results to 6BF5... No doubt something like 6CA5, 6CU5, 6HE5 etc would give similar results to the 6BF5 but I'd have to rewire socket to use those... As is, only wiring change I have to make from 6GY6 to 6BF5 or 6AQ5, is disconnect the normal audio input from pin seven and connect modulation transformer in series with plate B+ ...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Tue 14, 2017 4:31 am 
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Awesome


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Tue 14, 2017 11:22 pm 
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Well with B+ at 88v on plate and screen around 100v, a 6AQ5 performs as well as the 6BF5... At least by ear sounds good, demodulated audio waveforms are excellent... Again this thing fought me, for the third time no less.... Turns out the NOS 6AQ5 I was using has a intermittent heater to cathode short, ARGUUHHH!!! At least this time it wasn't operator error... Cured now, currently running a 6005... Reason for wanting the 6AQ5 to perform is it only draws approx 40% the heater current of 6BF5(450ma vs 1250), also fact plate current is probably half is a big plus...

Now I have details under control, in the next couple days I'll run through a audio sweep test...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Wed 15, 2017 5:17 am 
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Cool.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Wed 15, 2017 3:58 pm 
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Tube Radio wrote:
Cool.

Yeah but now I've added a complete audio amp, along with a second audio transformer just to more or less equal function of a 6888 & 6AV6 or other audio preamp tube... Would not be so bad if a cheap modulation transformer were available...


Ummm wonder how a small 1:1 120v isolation xformer would perform???

Problem is I don't know of one with a center tap, there goes the PP audio... :cry:


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Wed 15, 2017 4:33 pm 
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Agreed.

Wouldn't necessarily be a good choice for a part 15 AM transmitter unless one maybe had a HI-FI AM radio and the method of modulating the 6888, 6GY6, ETC... caused roll off of the treble.

Now if one wanted to drive a 1920's high impedance speaker a single 10DE7 with 6C4 as a preamp and a Hammond 600 to 4/8 ohm transformer as an audio choke with the speaker cap coupled to the plate makes for a nice amp for the antique speakers.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 1:20 am 
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Been a little cooler up in top of garage than I like so haven't done much, supposed to be 63* & sunny tomorrow, ought to be toasty upstairs...

I did check tube basing and found a 6DS5 is same and also 6AR5 except for no connection to pin 7... Problem is like other power pentodes I've tried some time back, they don't modulate as well as a beam power tube(6AR5 is miniature equivalent to 41/6K6)... The 6DS5 modulates nicely, apparently is the tube of choice for this application... Problem is I only have the one, so dunno if this particular 6DS5 is a fluke... Possibly can "fine tune" the 6AQ5 a bit further, up to this point I've mostly been troubleshooting and chasing my tail... It's run all day without issues, I'm almost afraid to touch it...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 2:31 am 
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Hi all. I have a transmitter using a 6AW8A as an oscillator/driver and final. Plate+screen of pentode is at 50V the triode is at 140V. This provides class C drive and the pentode modulates easily with a reversed output transformer that came out of a zenith console stereo many years ago and it had 6BQ5 in single ended circuit. The modulator used the other transformer with a 6AD10 as amp and output. very similar to a 6AQ5 and 6DT6 pair. The modulator and preamp were powered at a 250V level. This way there was ample power available to plate modulate the 6AW8A pentode.

Tetrodes do modulate easier as well as beam power tubes. a 6CL8 tetrode modulates well at low B+ of 50V as well as others like 6AN8 and 6U8A to mention a few. I used the 6AW8A because the triode gain is 70 so drive is good and I have lots of other tubes with the same pinout.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 4:18 am 
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Umm 6AW8 & 6AD10, that's a interesting setup... The 9DX base video amps are plentiful, I remember one having a gain of 110 on triode section...

The transformer I'm using to modulate my output is also SE 6BQ5... The 6DE7 amp uses a universal PP xformer, I believe rated for 15w or so... I tried different taps but the 7K plate to 8 ohm VC matching into the 6BQ5 xformer seems to be best...

I was thinking none of these tubes would perform very well at 50v on plate(actually concerned that 80v was enough) but results surprised me after B+ reduction for 6AQ5 when 6BF5 was installed... Was at 52v and the cathode was probably 5v or so(didn't measure it)... Demodulated audio waveforms looked fine... With 6DS5 current dropped enough that Plate voltage jumped to around 93v, again waveforms are fine...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 6:35 pm 
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Hi. The 6AW8A triode makes a good oscillator either coil cap vfo or xtal. It can also be used as a driver if a pll or other form of excitation is used. I use a vtvm to measure voltages since it has an isolation resistor in the probe tip to measure grid voltage. I fine the drive is not overly critical as long as the tube is driven to class C so the tube will function in the square law area as this provides the most linear modulation characteristic. with the R.F. choke tied between plate and screen will give good results as they will follow each other for full 100% modulation. The grid drive I find good is about 15v negative as a starting point. I have the triode setup in wide band mode meaning no tuned circuit in its plate just an R.F. choke of about 2.5MH and a 22k series resistor to B+. If a pc-70-osc coil is used with the tapped winding as the grid winding and a cap across it to resonate it and the other winding in the plate circuit of the triode instead of the R.F. choke and resistor then the plate voltage of the triode can be lowered further. In either method of drive making the B+ to the triode variable you can find the best drive for your version depending on your mod transformer and final plate loading circuit. Also the transformer used in the plate of the final as a modulation transformer should not be a push pull type but a single ended type as they are gapped and wont saturate so easily. The modulation amp can be either push/pull or se as the amount of power needed is small. A well matched setup with the secondary taps adjusted to give good match (dont bet on what the transformer windings are said to be experiment) should work with 6DE7 and other triodes as well. Note the quality of the transformers has a large effect on audio quality. The ones taken from an aa5 clock radio will have limited frequency response compared to a transformer from a quality console stereo with large speakers.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 9:15 pm 
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Well either the amp(probably) or xmitter is a bit lacking on bottom end, needs approx +4db increase to bring up 40-50 cps response, also requires around -2db decrease at 160cps(figures given from scale on EQ)... I'll check just the transmitter to find out whether amp or Tx is deficient... I seem to remember approx same figures when I checked the 6888 Tx a few years ago...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Thu 16, 2017 11:06 pm 
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First place I would look is at audio going into plate choke right off of modulation transformer. Either the transformer does not have a good low end or the dc is saturating it i am guessing. scope or vtvm should be able to read the ac at that point then do a sweep from 1KHZ to 50 HZ and see how it does.


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Fri 17, 2017 4:54 am 
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The transformers are loafing, with 6AQ5 or 6DS5 not even warm(was a slight temperature rise with 6BF5)... Per a RS audio wattmeter, 150mw fully modulates it... The modulation xfomer does load the output transformer somewhat, at same amount of gain into 8 ohm speaker, the meter is indicating 500mw... I'll try a lower impedance tap to see if that makes a difference...

The RF plate choke could be suspect, it's from a 1960s surprise box of obsolete components... They've measured around 1.5mh, but I installed a core that doubled the inductance(that may or may not actually be installed)... No doubt something to check on...


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 Post subject: Re: 10DE7 tube
PostPosted: Nov Fri 17, 2017 7:00 am 
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Location: pensacola fl
Yes I agree it is loafing but maybe the transformer's low frequency end from say 250HZ down to 50HZ is the culprit not heating. Remember that many output transformers had in their first lives as outputs and not modulation had capacitors across their primaries that in effect lowered their high end to help bring up the bass end. As a modulation transformer this would cause the transmitter to hit 100% modulation at the higher end and therefore the low end could never hit 100% at the same time or level point. It may be necessary to put a capacitor across the high impedance side of the transformer on the final side. I do not see the R.F. choke having any influence on the audio response as it is very low reactance at audio especially the bass end and the D.C. resistance is usually small as well and that is what the audio will see on its way to the final plate andthat is not in the circuit to the screen grid in pentode mode. Can you run a sweep on the transformer to see it's response curve? Then you can see whats up. Measure the audio at the modulation transformer's input winding and on the high side at the same time with it not connected to the final circuit and then do it again with it connected just to see the loading on it as well.


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