HQ-170A -- The Trouble With Tubes, Experts Wanted

[K7PEH]

I have a 1962 to 1964 vintage Hammarlund HQ-170A that seems to work fine.  I replaced some of the tubes today but in doing that discovered that one of the tubes seems wrong based on the circuit tube diagram.  No, it does not appear to be a standard equivalent and it is not even the same kind of tube.

This tube is used for the HQ-170A AVC circuit and also the AM Detector and 60 KC IF Amp.  Yes, it is three tubes in one.  A 6BV8 that is a triode and two diode tubes in one glass envelope.  However, that is not the tube in my radio.  I have a 6BQ7A which is a dual triode tube -- not the same kind at all except for the fact that they are both 9-pin and use exactly the same kind of tube socket.

When I put the 6BV8 into the socket which is the tube that should be there, the radio does not work.  Well, no signal at all.  But, the 6BQ7A does work and the radio seems to work and all features work (including AGC, mode selection, and so on).

I have not opened up the cabinet but I am guessing that maybe someone modified the circuit for some reason.  Or, maybe this is a standard but odd configuration although the tube diagram plastered on the back of the cabinet shows the tube that should be there, the 6BV8.

So, anyone heard of anything like this?  Maybe this is some well known mod to the HQ-170A.  But, like I said, the radio does work with the exception that the audio seems week but that may be low sensitivity and indicating that alignment might be needed.

Thanks for your help.

[KB4QAA]

Of course.  People have modified radios all the time and substituted different parts!

Only you can answer the question by opening the radio up and comparing it against the standard schematic.

[AC5UP]

I have a strong hunch the detector was modified for SSB reception........ Not uncommon at all.

BTW: If you have any old Tek 500 series scopes in your junque pile, liberate a 6DJ8 and try that in place of the 6BQ7A. Supposed to be a higher gain, lower noise version of the same tube. As a detector you won't hear any difference, but who could ignore a chance of higher gain with lower noise? Another curious factoid I learned while looking that up is that the 6BQ7A is essentially a 12AU7 with a six volt filament.

Will wonders never cease?

[K7PEH]

OK, I give up -- why would someone modify the HQ-170A for SSB reception when it is already a very good SSB receiver (as well as CW and AM)?

[KX5JT]

OK, I give up -- why would someone modify the HQ-170A for SSB reception when it is already a very good SSB receiver (as well as CW and AM)?

Some people love to tinker.... even when it's not broken, they have to fix it.

[KA5N]

The usual change in the 1960's era was to replace the diode detector with a product detector
in order to improve SSB reception.  A product detector is just a mixer which takes the IF
signal and mixes it with the BFO signal and outputs the audio difference in the two signals.
Of course you can use a simple diode detector to receive both AM, CW and SSB.
So if a product detector was substituted, the question is how good a job did the previous
owner do. 
I have found that more often than not mods are poorly done and often make performance
worse rather than better.  Sometimes they work marvelously well.  With used gear "you
never know what you are going to get."   Forrest Gump said it first.

Allen

[G3RZP]

The 6BV8 wasn't that common a tube over here, and was actually quite hard to get. In my father's HQ170A, one of the diodes went: I took a normal silicon diode (1N914 type) and wired it across the pins of the 6BV8 and we were back in business. It could well have been something similar that led to the 6BQ7 being put in. Maybe a 'Use what you have'......

One simple mod especially if you wanted to make full use of the 6 metere band on it was to clip off one cathode pin of a 6AK5 - the one corresponding to g3 of the 6BZ6 - and plug that in the RF amplifier position for a worthwhile improvement in sensitivity on 6 and 10. Not so good on the lower bands for cross modulation effects, though.

As it got older, we found that especially on 6 and 10, the 6C4 oscillator tube become critical, often refusing to work. Also, be warned - I've seen eastern European manufactured '6BA6' branded devcies that had g3 and cathode internally connected.

[K7PEH]

KA5N -- well, the HQ-170A already has a product detector.

But, the idea that replacing the two diodes of the original 6BV8 with solid-state diodes and then replacing the tube with something simpler might have merit.  It will have to wait until I have time to crack open the under-belly and take a look.

[AC5UP]

OK, I give up -- why would someone modify the HQ-170A for SSB reception when it is already a very good SSB receiver (as well as CW and AM)?

When you say the HQ-170A has a very good SSB receiver, are you basing that on the performance of a radio that has been modified?

For all you know, the mod in yours really does work better than the box stock factory setup............

[K7PEH]

When you say the HQ-170A has a very good SSB receiver, are you basing that on the performance of a radio that has been modified?

For all you know, the mod in yours really does work better than the box stock factory setup............

When I made the comment about the HQ-170A as a good SSB receiver it was a comment about the HQ-170A.  Actually, I bought an HQ-170AC brand new in 1965.  One of the bigger mistakes I made in life was selling it in 1968 to gather tuition money for college (my junior year and I was short a couple of hundred dollars).  But, the SSB reception of the HQ-170A ranked up with the best of them.

Now, these changes involving this tube do not deal directly with the SSB features of the HQ-170A.  I am suspecting that the best explanation for the mod is to replace the diode portions with solid state due to having to replace the 6BV8 tube with something else that did not have diodes.  This is my top guess at the moment but there must be diodes down there underneath the cabinet since they certainly are not in the tube now used.

[KE9UA]

Hello Everyone,

I am working on a HQ-170A for W9ZWT and replaced several of the 6be6 and 6ba6 tubes plus the 6c4 osc tube and went though a complete alignment of the if's and rf/osc stages and the radio is working ok on all the bands, but the audio of the ssb signals is distorted and its not in the audio stages as a tone injected at the volume control wiper is clean and clear. The radio has good sensitivity other than the signals received are garbled kind of like hearing someone talk though a fan. Any clues guys on what to look for here. All the rf and front end tubes are new and tested on a hickok 534b tester and are all strong with no shorts or gas problems. This radio sat in his garage for a number of years and he's 93 years old and wants to use this again. I'm stumped for now and could use your help. Tnx and 73

Dennis

[KA5N]

Since you have checked the tubes and done the alignment, the next thing would most likely
be bad capacitors either audio coupling or bypass.  Also there could be something awry with
the AGC (or AVC as it used to be called).

Allen

[K7PEH]

Since you have checked the tubes and done the alignment, the next thing would most likely
be bad capacitors either audio coupling or bypass.  Also there could be something awry with
the AGC (or AVC as it used to be called).

Allen

Allen, I am not sure I understand the nature of your comment.  I am not particularly trying to restore this receiver, just replaced some tubes and discovered the odd tube placement which is the reason for my original question.  Actually, the HQ-170A functions quite nicely right now -- not as good as my K3 for sure but good enough for an old thing.

I did find out the nature of the odd tube -- as someone suggested it appears that some previous owner could not replace the original tube so put in a single triode tube to replace the triode plus diode plus diode combo and then modded the circuit underneath putting in two solid state diodes in place of the tube diodes.  I have not looked into how these solid-state diodes would compare to the functioning of the original but it does work.

[N4NYY]

For all you know, the mod in yours really does work better than the box stock factory setup............ Tongue

I am a tube expert. Let the flaming begin! LOL

[KA5N]

Allen, I am not sure I understand the nature of your comment.  I am not particularly trying to restore this receiver, just replaced some tubes and discovered the odd tube placement which is the reason for my original question.  Actually, the HQ-170A functions quite nicely right now -- not as good as my K3 for sure but good enough for an old thing.

I did find out the nature of the odd tube -- as someone suggested it appears that some previous owner could not replace the original tube so put in a single triode tube to replace the triode plus diode plus diode combo and then modded the circuit underneath putting in two solid state diodes in place of the tube diodes.  I have not looked into how these solid-state diodes would compare to the functioning of the original but it does work.
[/quote]
I never suggested that you should change all the capacitors, just that the problem you
described might be caused by a bad capacitor. 
For example a bypass cap on the AVC line that is open can cause strange effects in the audio.
A leaky capacitor can muddy the audio.  The notion that all caps be changed is not a good
idea most of the time.  Many caps are good for 50 or more years and others fail in a short time. It is cheaper to find the bad components and replace them than to shotgun the whole set.

Allen