Started a Rowe R-4359A amp conversion

Schmidlapper

Super Member
I have been doing lots of initial research and have come down two options regarding output transformer secondary taps. The 7868 tube data sheet says in Class AB push pull the optimal OT reflected impedance is 6500 ohms, and based on the R-4359A OT secondary taps turns table created by Secret4mula linked.
The usable combinations that I found are listed on the cleaned up schematic.
ami-rowe2620ampcleansch_zpsbed378fb.jpg
For a 8 ohm load, I could use the E1 to E4 tap for 5800 ohms with E4 already common, or I could use E1 to E3 and get 6800 ohms and move the ground wire from E4 to E3. Am I correct here that I can just move the grounds from tap E4 to E3 without affecting the feedback since it comes off E1? Is it the better choice here to get as close to the optimal 6500 as I can, if it goes over just a little? I know the obvious downside is slightly lower output wattage. So here I am looking to the community for some input?
 
Well after more in depth re-reading, I found where Gordon had already covered this awhile back. I'm encouraged that my own thought process had come to the same conclusion. Thanks Gordon. One question not covered though, do I still need to move the ground if I use other taps or can I leave it on E4 regardless?

Given that the B+ voltage on the amp is only 365v, this might be a good idea.

this amp has an output transformer that should be easy to re-optimize to different screen voltages.

Since the turns ratios are given for many combinations of taps on the schematic posted earliest in this thread, that could be done by the following tap combinations:

For 4 ohm speakers: Try either E2/E5 (about 7400 ohms) or E2/E5 (about 11600 ohms)

For 8 ohm speakers: Try either E1/E3 (about 6700 ohms) or E2/E6 (about 10K ohms).

For 16 ohm speakers: Try either E1/E6 (about 7700 ohms) or E1/E4 (about 11600 ohms).


Regards,
Gordon.
 
Leave it on E4 - neither speaker terminal need to be at ground. Moving the ground will change the amount of feedback.
 
The main differences between the schematics I have found online and the amp I have is the power supply, the R-4359A has a different circuit after the Diode and the 100ohm resistor that is not shown anywhere in the R2620 drawings. It consists of one orange transformer tap lead going to a 500Mfd and another going to a pair of 500Mfd caps and two or three diods. from what I can tell the one going to the pair of 500Mfd caps may not be in the stand alone amp circuit. Anyone already sort out what can be remove?
 
IMG_0444_zps2tnkxwwh.jpg

Here is a photo of the unit, with the output transformer end bells off to let them dry out, just incase they got wet while washing the chassis down. The transformers taps on this unit all ohm out as normal and there is no burnt smell in either of them. The unit was pulled straight out of a supposedly operational jukebox by me, but I noticed that both output tubes on the one channel have burnt 1k resisters between the grid pins 8 and 1 the B+ going to the center tap of the output tranny (some similar units don't have these on print and use pin 7 to 1) Not sure what that indicates happened, but I was concerned about the transformer of course. The resistors may have saved them.
 
Measured my output Transformers today and it is a little different from the R-2620 in this way also. The best combination of taps I could find was 26.8 turns on E1 to E3 for a 5703 ohm reflected load impedance. Taps E1 to E4 was 25.2 turns, so lower. All other tap combos were either lower or much higher than the recommended 6500 ohms.
R-4359A%20Output%20Tran%20Turns_zps6hyxqol9.jpg
 
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Hey Bill,

Looks like you're well on your way to having your Rowe amp up and running! For the record, I've retained my 'ground' circuit using the E-4 tap wire on the OT, and then done listening tests using both the E-1 and E-4 tap on my 8 ohm speakers. I know this is not as scientific as 'doing the math'.......but I have to point out something here: As you probably ran into when doing your Maggie project, the Maggie OTs are pretty much all 'optimized' for a 4 ohm load. However, if you're like me, most of my speakers are all 'rated' as being 8 ohms. So, the poor little maggie is seeing essentially double the resistance load that it's 'supposed' to see in order to reach it's proper output potential. Yet, in doing some further checking with many of the experts here on AK, it seems the use of the 8 ohm speakers does two things: 1) reduces the output potential in terms of available wattage, and 2) also reduces the distortion associated with the available output. So, in listening to the Maggie with 8 ohm speakers...... it actually sounds wonderful! Well, likewise the Rowe amp with it's not-quite-perfect match to an 8 ohm load. I think in using the 5.7 or 5.8 K taps for your 8 ohm output, you perhaps DO lose a bit of potential output power. But, the amp sounds WONDERFUL when using that configuration!

As I think I may have mentioned, I have now re-purposed several of these from Juke Box configuration to 'home audio' power amps. I've even pulled some of the 9-pin sockets and replaced them with octals and converted the amps to 7591. While that hasn't really produced any distinguishable 'sonic' difference...... it does allow the usage of currently produced tubes now available at some reasonable prices. I'd note that the 'fixed' bias circuit in this amp makes it a great candidate for reconfiguration to an IBAM (Individual Bias Adjust Mod)..... and I believe there are several circuits for that here on AK. I've converted mine over to 'adjustable' bias......but inserting a single pot in the circuit allowing me to adjust an entire quad.....and in using well matched tubes I've not had a problem in getting my amp to bias properly on all four output tubes. But, I think the IBAM is well worth it.....and my next one will have that!

I'll be honest: I think this is one of the best sounding amps I've had the pleasure of listening to. I have a few Scott and Fisher amps that are 7591 based units, and they are what got me 'hooked' on that output tube. I've got an old LK-72, a 299D, and a Fisher 500....all of which are wonderful sounding amps and have more than adequate power IMHO. Does the Rowe sound as good as those amps......well, hard to tell. I've never actually sat down and done an A-B shoot out! But, as a daily driver, my Rowe conversion sounds great. I hope you enjoy yours as much as I do mine. Good luck.........and keep the thread updated as you progress with this project.

Tom D.
 
Well I have pretty much gotten all my answers and my research complete and have begun slogging through the process of rebuilding now. I am using the electrical schematic found in the Warfcreek thread documenting his and secret4mula's builds which incorporates the IBAM mod.

Current build status,
I have ordered and received all new components found on the schematic with only the trannys, chassis, tubes and five tube sockets remaining original to the unit.

All new holes and openings have been added and any unneeded holes patched.
48a49c5d5831c65a1d28eebe644d7172_zps7oagzynk.jpg

Paint application is next up and cleaning up all the old solder tabs and tube sockets in preperation for the new parts.
 
Bill,

Just a couple of quick suggestions, as well as one comment:

First, (the comment), I believe regarding the feedback situation, you don't need to change the location and circuit of the feedback even if you chose to use other taps for your final output. Case in point; there are a number of amps (with the Dynaco ST-70 being one that comes to mind first) where the feedback is taken off the 16 ohm tap and the speakers are run from the 8 ohm tap. In the case of the Rowe amp, the distinction would be that you use the E1 and E3 taps for speakers and keep them 'insulated' from the chassis. This would leave the E4 tap still chassis grounded, and E1 supplying the feedback to the driver. Try the amp with your 8 ohm speakers, and then try it with the same 8 ohm speakers on E1 and E4. Pick what you like best, and go from there.

Also, you've done a LOT of chassis prep, but retained the mounting ears used to locate the chassis into the Juke Box. You may want to grind off those rivets now so as to remove those tabs. And, lastly, regarding the RCA inputs, I found the location to be really less than desirable. So, you may want to consider a change of input location now vs after you do all the cosmetic work. Again....just my thoughts, but figured I'd share them now while you still have the opportunity to make any changes you may decide are appropriate. WC
 
Update on progress.
IMG_0482_zpsezakyudq.jpg

IMG_0481_zpsqw6bukoq.jpg

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The plan is to maintain the general look and feel of the original jukebox amp, think Hot Rod Jalopy. I will use the mounting tabs as a solid location for some chassis feet I removed from an old Heathkit receiver. I also intend to incorporate the loop handle and wire guard bail, I find them very useful when working on the amp upside down.
 
I'm thinkin' 'Rat-Rod'...........lol

So, colors? How 'bout a nice gloss black on the chassis, and a bright canary yellow for the transformers? You could even do some black racing strips on the iron!!......lol

Good idea on the tabs, handle, and wire guard bail. You're on a roll......... WC
 
waiting with baited breath....... WC

Can you clear up a question that I have on the schematic, they have the 100uF cap drawn in backwards to ground on the original schematic as well as the redrawn ones, is that correct? It is used to for the bias voltages.
 
If the bias voltages are NEGATIVE, the correct orientation for the cap would be + grounded.
 
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