Scout 20 4 driving 4 amplifier

[Unit 75]

Does anybody happen to have a schematic for this amplifier? I searched the net and can't seem to find one. Thank you in advance.

 

[AudioShockwav]

Is that the one with 4 smooth tops and 4 6kd6 tubes?
I had some stuff on SOS electronics somewhere
I will go look

73
Jeff

 

[fixer]

As far as l know, there are no schematics available. Tube set up is the same as the D&A Maverick..
What do you need to know? I have repaired many of them and have experience plus many notes on them.

 

[Unit 75]

The problem is audio. Previous owner had the HV supply caps replaced 2 weeks ago. Amp loads and tunes fine with full output power. It has proper biasing and the plate choke is in good shape so there should be no feedback into the HV supply. The amp is being driven by a Cobra 148GTL, which should not overdrive this amp configuration and yes the limiters are still intact in the radio. Radio is keying 3.8 watts on AM swinging to about 7 watts...no more. Tubes are lighting up evenly. I'm suspecting something is causing tube oscillation because audio is sounding distorted and underwater. That is all the info I can offer at this time. Thanks.

 

[nomadradio]

distorted and underwater

Sure sounds like a failed filter capacitor in the negative grid-bias voltage source. Did all the small electrolytics get replaced along with the big ones? The only rational approach is to replace every, single one, large and small both.

73

 

[Unit 75]

Yes. All electrolytics were replaced 2 weeks ago. They are all 105c caps. If one of the filter caps in the negative grid-bias voltage source failed you would not have full output power from the amp. This has full output.

 

[fixer]

Could one of the grid bias caps, usually 220 uF, possibly be installed backwards? Positive would go to ground.

 

[Unit 75]

That doesn't make sense. That short would be evident as soon as you turned the amp on. If by chance it didnt blow up, it would cause the tubes to red plate from excessive current draw and you definitely would not have full power output. Thank you all for your input. I will continue on with this amplifier and report my findings once I figure it out.

 

[nomadradio]

Is the driver's Plate Tune control a bit "twitchy"? Does it behave differently on one side of the knob's peak position than it does on the other side of the peak? Next suspect on the list after a grid-bias fault would be a stability issue, especially in an amplifier with a built-in driver stage. Maco amplifiers are famous for this problem. We routinely place a bit of hardware-store screen between the driver tubes and the final tubes.

Like this:

Likewise, the coax from the final-stage Load control to the antenna relay should have the shield grounded. Maco would leave the shield "floating", not connected at all. Those two "upgrades" are mandatory for any Maco that we work on,

73

 

[Unit 75]

Is the driver's Plate Tune control a bit "twitchy"? Does it behave differently on one side of the knob's peak position than it does on the other side of the peak? Next suspect on the list after a grid-bias fault would be a stability issue, especially in an amplifier with a built-in driver stage. Maco amplifiers are famous for this problem. We routinely place a bit of hardware-store screen between the driver tubes and the final tubes.

Like this:

Likewise, the coax from the final-stage Load control to the antenna relay should have the shield grounded. Maco would leave the shield "floating", not connected at all. Those two "upgrades" are mandatory for any Maco that we work on,

73

Nomad....That's why I like you. You think outside the box brother. I know something is causing oscillation in the tubes just couldn't put my finger on it. I think you nailed it cause I can honestly say that I can't see anything wrong with this. I'm going to install the shield. P.S. tune control is not twitchy at all. The amp loads and tunes fine. I will hit you back after I install the shielding bro. '73