NAD Vox Buckingham - Couple of issues

[52 Bill]

As far as I can remember, Thomas Organ only used 8 and 16 ohm drivers in their cabinets. Though they may have used 4 ohm speakers at sometime.

The reverb tank on this model is transformer driven, so I think that the tank you need is a 4AB with a grounded input and output jack. What resistance does the input coil of the existing tank read?

The distortion/fuzz is the only effect that needs the foot switch to operate. It only works on the normal channel. You can test it by shorting pin 2 of the DIN jack to ground.

[Mamm7215]

As far as I can remember, Thomas Organ only used 8 and 16 ohm drivers in their cabinets. Though they may have used 4 ohm speakers at sometime.

The reverb tank on this model is transformer driven, so I think that the tank you need is a 4AB with a grounded input and output jack. What resistance does the input coil of the existing tank read?

The distortion/fuzz is the only effect that needs the foot switch to operate. It only works on the normal channel. You can test it by shorting pin 2 of the DIN jack to ground.

I'll attach a pic of the label, definitely states 4 ohm load. I read somewhere the AB designation was for tube amps but looking at the Amplified Parts page it seems it refers to input/output impedance so 8/2250 would seem right. I'll meter the input jack. Thanks again for your help.

PS don't know why it shows up sideways...

[52 Bill]

I'll attach a pic of the label, definitely states 4 ohm load. I read somewhere the AB designation was for tube amps but looking at the Amplified Parts page it seems it refers to input/output impedance so 8/2250 would seem right. I'll meter the input jack.

Yes, the entire cabinet is rated at 4 ohms, but the individual drivers could be 4 or 16 ohms to get there. You need to test the individual speakers to see what you have in there.

Remember that when reading the coils in the tank, impedance and dc resistance are not the same thing.

[Mamm7215]

I'll attach a pic of the label, definitely states 4 ohm load. I read somewhere the AB designation was for tube amps but looking at the Amplified Parts page it seems it refers to input/output impedance so 8/2250 would seem right. I'll meter the input jack. Thanks again for your help.
[ATTACH=CONFIG]50855[/ATTACH]
PS don't know why it shows up sideways...

Input and output jack measure 175 Ohms. Tried getting my probes to contact somewhere on the secondary but couldn't really find a decent reading. The pan in general looks to be in pretty bad shape so another one is in order no matter what, I think. Also found one of the reasons for intermittent reduced volume and scratchiness was the 3-pin to 1/4" adapter for the speaker out likely is dirty and/or poor contacts so will have to do more cleaning and possibly re-soldering some leads.

[52 Bill]

Input and output jack measure 175 Ohms.

That would indicate a high impedance input tank, like the ones in the later versions of that amp. It may have been changed at some point in time, who knows. If the reverb is transformer driven, I still say it should be a low input impedance tank.

Yes, clean every contact, switch, jack, etc., etc. with DeoxIt. Doing this will often solve a host of problems in any amp.

There are stickers on your speakers, what part numbers are on them? I think they will start with 33-something. I used to have a list of T.O. part numbers that may list your speakers. There will also be Oxford codes on the same labels that will suggest a model and manufacturing date.

[Mamm7215]

That would indicate a high impedance input tank, like the ones in the later versions of that amp. It may have been changed at some point in time, who knows. If the reverb is transformer driven, I still say it should be a low input impedance tank.

Yes, clean every contact, switch, jack, etc., etc. with DeoxIt. Doing this will often solve a host of problems in any amp.

There are stickers on your speakers, what part numbers are on them? I think they will start with 33-something. I used to have a list of T.O. part numbers that may list your speakers. There will also be Oxford codes on the same labels that will suggest a model and manufacturing date.

The numbers on each speaker labels are (left side)
33-5100-6X
465-941
12T6-12
The two speakers on the right side are slightly different with 465-945 being the 2nd number instead of 465-941. I think the 941 and 945 correspond to the 41st and 45th week of 1969. 12T6-12 is an 8 ohm speaker, I believe. The Sovereign V417 cabinet is a bass cabinet. It didn't come with the head, I've had it sitting around for years. Sounds ok though. I do have a spare greenback kicking around I wouldn't mind putting in a cabinet, might go good with the head...

So do you suspect the reverb tank isn't correct? It sure looks like it came with the amp...

[52 Bill]

You are correct about the speaker date codes. Wired as it is, the cabinet is an 8 ohm load.

The tank is an original Vox tank, I just don't know if it is original to the amp. Are there any code numbers on it anywhere?

[Mamm7215]

There’s a label but it’s too faded to read. I didn’t see anything stamped in the metal.

[J M Fahey]

Unsolder one wire from a speaker terminal so itīs free, unconnected to anything else, and measure DC resistance terminal to terminal.
Previously short meter probes together, they will usually NOT show zero but some other value, say 0.8 ohm or so, substract that value from what you measure at speaker terminals.

Then plug a spare plug into cabinet back and repeat above test.
IF you use a whole plug to plug cable, you might easily be adding 0.5 to 1 ohm resistance in series, clouding proper measurement.

Post all results here.

[Mamm7215]

I’ll have to try this later, I’m away from my studio tonight but I’ll do that. I do know when I plug a speaker cable into the cab and measure the resistance at the other end’s jack (tip-shield) I see 8.1 ohms so that’s what the amp sees when plugged in unless I’m missing something.

[g1]

Also verify that sound is coming out of all 4 speakers, or check the resistance of every one, using procedure JM stated in post #24.
A couple non-working speakers could make a 4 ohm cab measure 8 ohms, and the sound difference is not always obvious with a 4x12 cab.

[loudthud]

As far as I can remember, Thomas Organ only used 8 and 16 ohm drivers in their cabinets. Though they may have used 4 ohm speakers at sometime.

Cambridge Reverb V1032 used a 4 Ohm 10 inch speaker. Only 4 Ohm speaker I know of. Buckingham used two 16 Ohm speakers in parallel.

[Mamm7215]

Also verify that sound is coming out of all 4 speakers, or check the resistance of every one, using procedure JM stated in post #24.
A couple non-working speakers could make a 4 ohm cab measure 8 ohms, and the sound difference is not always obvious with a 4x12 cab.

Sound is coming out of all 4 speakers, I just verified this now (got done early) and volume is the same for all as much as I can tell by ear. Next time I have it apart I'll do a more thorough measurement but seems like the cab is ok at this point...

[Mamm7215]

Unsolder one wire from a speaker terminal so itīs free, unconnected to anything else, and measure DC resistance terminal to terminal.
Previously short meter probes together, they will usually NOT show zero but some other value, say 0.8 ohm or so, substract that value from what you measure at speaker terminals.

Then plug a spare plug into cabinet back and repeat above test.
IF you use a whole plug to plug cable, you might easily be adding 0.5 to 1 ohm resistance in series, clouding proper measurement.

Post all results here.

With wires off terminals all 4 speakers measure 3.4-3.7 ohms across the terminals. So I guess that makes it a true 4 ohm cabinet. Since they're 4 ohm drivers instead of 8, even if I put a 4 ohm load in series to make 8 total at the jack, is it still not a good idea since they're 4 ohm speakers?

[g1]

That will work. But it will get half of the power coming from the amplifier so make sure it has a sufficient wattage rating.