Rowe/AMI Tube Amp Recapping..

Hey guys -
Ron, per your email, I'm moving the thread here, from the Jukebox list.

I'm assuming this is because John isn't a member here, and you want to get my business of rebuilding my amp from him

(Seriously, I don't care where the info's posted - email would be fine as well! However, I recognize the benefit of having it publicly posted, so that others can search and learn..!)

So, this is about the last Rowe/AMI model tube amp, the R-4359A.

To recap (pun intended) - Now that I recently got myself an MM6 back, I want to slowly work on it and get it back to its best possible condition. I have a feeling that this will mean that the Amp needs attention.

As it stands right now, there was a very loud raspy hum from the amp - I was able to fix this, due to a bad ground. (In several places, including the AC plug, go figure!)

The amp now sounds okay, but pretty bad. It's not helped by the fact that the pots in the pre-amp are raspy as heck - trying to adjust upper cut-off and bass boost results in a LOT of crackly static, and just touching the pots will adjust the audio in one way or another (depending on how, where, and the pressure in which you touch them.. I'll be looking at the pre-amp later.

The amp still has original tubes on it - up until I cleaned them off, they have the red/pinkish 'Rowe AMI' logo on them. Unfortunately, the amp hadn't been cleaned since 1972, when it was installed. The tubes were caked with fried dust, and the inside of the amp was filled with black messy dust coating everything. I sucked it all out with a brush on my shop vac, and everything passed the visual check I gave it. The tubes are very cooked - but they all glow, and I get audio out of both channels.

Volume control works - but there is a noticeable delay. Turn the vol knob, and after about two seconds, the amp responds. I don't *really* consider this an issue, just stating so that it's known, in case it could give a hint as to other things.

I'd like to get it recapped, and get new tubes for it, if necessary. However, I'm not in a hurry to do so (as .. it's been a long time since I've had it, and have a lot of music to listen to!). I don't MIND sending it out to have someone do it (Ron?) - my biggest concerns are shipping something this big and getting it damaged, and the time for rebuilding. (Again.. i wanna listen to music!)

The amp DOES have one odd behavior, however, and it may be tube related. Out of the left speakers, at times, there is a very soft .. sound. It's very hard to describe, and it varies, but sometimes sounds like someone rubbing skin over a wet balloon. (Like a floating toy in a pool..) It doesn't happen all the time and I do not know if the sound is dependent on the volume. (Right now, the Juke is turned up semi-loud, and the noise is barely above a whisper.) It *DOES* happen while the amp is muted, however!

So the last I saw on the Jukebox list was checking some voltages. I will likely do this tonight or tomorrow night (again, I'm in no rush), and post findings here.

Thanks in advance all -
-Mike

Mike, I'm "Rob" -not Ron. Hopefully, he is around too

As I mentioned, the preamp card must be washed and the controls worked w during that process. The electrolytic caps need to be replaced and as I mentioned, I suggest eliminating the edge connector and soldering the wires directly to the board.

The slight delay in volume control action is normal. the system controls volume via DC in the control and that is buffered with a capacitor.

I suggest taking both amp and pre box out of the machine and onto a work surface. then measure the voltages on the pins of the power amp and and check the aforementioned 100 ohm resistors. You can attach regular 8 ohm speakers to the E1 and E4 terminals for full 25 watt output and a regular turntable w/magnetic cart can be used. It is not necessary to reverse phase the pickup cart here since there is no woofer across the channels. Just reverse one pair of speaker leads.

Here is a wattage chart for the Rowe tube amps:

http://s1192.photobucket.com/user/Rob-N ... sort=2&o=7

As I mentioned, the bias supply of around -32 VDC is critical in saving the output tubes and changing-adding filtering to that would be a good idea.

Rob/NYC

Rob-NYC wrote:Mike, I'm "Rob" -not Ron. Hopefully, he is around too

D'oh! Pardon, I'm bad with names. Call me Steve. I split up your post to ask a couple of follow-up questions..

Rob-NYC wrote:As I mentioned, the preamp card must be washed and the controls worked w during that process. The electrolytic caps need to be replaced and as I mentioned, I suggest eliminating the edge connector and soldering the wires directly to the board.

The slight delay in volume control action is normal. the system controls volume via DC in the control and that is buffered with a capacitor.

Interesting - would it be a problem if there WAS no delay? I'm only asking because on my previous MM6 that I owned - way back in the late 80's to the late 90's - it's volume adjust was instant. This is why I thought that was odd. However, if that's normal, then that's good to know.

Rob-NYC wrote:I suggest taking both amp and pre box out of the machine and onto a work surface. then measure the voltages on the pins of the power amp and and check the aforementioned 100 ohm resistors. You can attach regular 8 ohm speakers to the E1 and E4 terminals for full 25 watt output and a regular turntable w/magnetic cart can be used. It is not necessary to reverse phase the pickup cart here since there is no woofer across the channels. Just reverse one pair of speaker leads.

Alright, I'll give this a try this week. To clarify - the amp will be okay if it ONLY has speakers connected to the extension terminals? (I've seen a fair share of amps in the past that needed to have a load on their main output..)

Rob-NYC wrote:As I mentioned, the bias supply of around -32 VDC is critical in saving the output tubes and changing-adding filtering to that would be a good idea.

Rob/NYC

Thanks Rob. I'll jump on this this week and report findings back. Since I can't play anything until my 'new' turntable arrives.

--Mike.

.....my previous MM6 that I owned - way back in the late 80's to the late 90's - it's volume adjust was instant.

that may have either been due to loss of capacitance in the cap that damps the DC shift, or it may have been due to the way the circuit works. To control the volume with DC -and- provide a degree of loudness compensation, the system essentially separates the lows and highs into two feeds going into the diodes that control the level. In the first approx 2/3 rotation of a typical pot the voltage applied to the diodes begins to go down. At first the voltage lowers most on the diodes controlling the "lows" and finally it goes down on the "highs" That last reduction makes a bigger apparent difference in level. So if your preamp's gain was low due to fading capacitors or low voltages the volume control will normally live nearer to the last part of the rotation and thus come out of bass boost quicker. It is a matter really of perception.

To clarify - the amp will be okay if it ONLY has speakers connected to the extension terminals? (I've seen a fair share of amps in the past that needed to have a load on their main output

Any load -is a load. the only reason for connecting machine speakers was in amps where there was a jumper in speaker plug to disable the B+ if the plug was not there. Rowe doesn't do this.

BTW: You will need to connect a volume control. This is normally a dual 10K linear but just about any will do for tests. You can just remove the one from the machine as well.

Rob

"BTW: You will need to connect a volume control. This is normally a dual 10K linear but just about any will do for tests. You can just remove the one from the machine as well."

The volume control used on a Rowe Ami of this period is a 10k log control, it may be wired as a two wire or 5 wire (with a screen) dependant on the pre amplifier used.

Regards
Alan

Alan Hood
ami-man
UK

Not trying to hijack the thread but I too have always enjoyed 'Rock and Roll Part 1' more.

Rob-NYC wrote:that may have either been due to loss of capacitance in the cap that damps the DC shift, or it may have been due to the way the circuit works. To control the volume with DC -and- provide a degree of loudness compensation, the system essentially separates the lows and highs into two feeds going into the diodes that control the level. In the first approx 2/3 rotation of a typical pot the voltage applied to the diodes begins to go down. At first the voltage lowers most on the diodes controlling the "lows" and finally it goes down on the "highs" That last reduction makes a bigger apparent difference in level. So if your preamp's gain was low due to fading capacitors or low voltages the volume control will normally live nearer to the last part of the rotation and thus come out of bass boost quicker. It is a matter really of perception.

Any load -is a load. the only reason for connecting machine speakers was in amps where there was a jumper in speaker plug to disable the B+ if the plug was not there. Rowe doesn't do this.

BTW: You will need to connect a volume control. This is normally a dual 10K linear but just about any will do for tests. You can just remove the one from the machine as well.

ami-man wrote:"BTW: You will need to connect a volume control. This is normally a dual 10K linear but just about any will do for tests. You can just remove the one from the machine as well."

The volume control used on a Rowe Ami of this period is a 10k log control, it may be wired as a two wire or 5 wire (with a screen) dependant on the pre amplifier used.

Thanks for the info guys. No work yet, due to issues at my wife's work, keeping me busy after I get home. But hopefully soon I'll be able to get some voltage readings!

I'll just take out the pot in the juke for testing - this way I don't introduce any 'gotchas' once I put it back in (i.e. Amp reads fine connected to a new pot, but back in the juke and get something unexpected.

MrGorf wrote:Not trying to hijack the thread but I too have always enjoyed 'Rock and Roll Part 1' more.