Philco Integrated Amp conversion to Preamp help

[Harlon]

Philco Integrated Amp Conversion To Magnavox 9304-20 Preamp

I have a Philco K1629 and Magnavox 1ST622 which were my wife's step grandmothers who recently passed.

I have been using the Philco as a learning tool to get myself ready for the Magnavox refurb. Currently, I'm using the Philco to listen to an iPad and the TT that was part of the console. I've recapped and replaced the plug wire as well as removing all the unwanted radio parts. After starting the Magnavox refurb I cannibalized most of the electrolytics and made a failed attempt at the preamp conversion. In the meantime I damaged the choke on the Magnavox and have reassembled the Philco so I have something to listen to while I wait for it's replacement.

I was actually very proud of how quickly I was able to rewire and recap the Philco. For those who followed along on my Magnavox recap thread there was in fact no damage to the electrolytics. It's still the Philco but it sounds great, especially running through the Magnavox speakers.

To my inquiry, would it be possible to cut down the Philco further and utilize the preamp stage as a preamp for the 9304-20 and Micromatic TT. I thought it would be as simple as disconnecting the preamp outputs running from the balance pot to pin two on the output tubes. Unfortunately that was a failure. Along with a few other options I tried.

I've attached a photo of the amp section of the schematic in SAMs and the one inside the console.

Any assistance would be greatly appreciated.

 

[Harlon]

The other schematic.

 

[cademan]

You will want to take your preamp tap from the Philco at pin #2 of V7. The other tap at pin #7 of V7 for the other channel. Specifically, both the center lugs of the volume control can be used as a stereo preamp tap.

Don't connect an input directly to the output tubes at pins #2. Use the grid inputs (pins 2 & 7) of the 12AX7 tube.

 

[Harlon]

You will want to take your preamp tap from the Philco at pin #2 of V7. The other tap at pin #7 of V7 for the other channel. Specifically, both the center lugs of the volume control can be used as a stereo preamp tap.

Don't connect an input directly to the output tubes at pins #2. Use the grid inputs (pins 2 & 7) of the 12AX7 tube.

Will tying in at this point still allow me to utilize the volume, tone and balance pots for the new output amp? Sometimes it feels like these circuits work back to front or ambiently rather than a linear fashion.

Also, by tapping into the preamp, am I leaving the circuit intact and just soldering in leads for the new RCA plugs to pins 2 and 7 on V7. In my initial experiment I broke the circuit after the balance pot and used the leads that formerly went to #2 on the output tubes as sources for the new RCA plugs.

Thanks for the help.

 

[cademan]

If you are wanting to use your Philco as the preamp with the tone control to drive the Magnavox amp, it looks like you will have to tap in at the junctions of R33 and C44 for the one channel, and R41 & C50 for the other channel of your Philco, and then from there, wire them directly to pins 2 of the 6BQ5 tubes of the Magnavox amplifier.

Be sure that your Philco chassis is connected (grounded) to the Magnavox chassis. Sometimes the RCA patchcords will work in connecting one device to another but if they are not both tied to center tap (basically chassis ground), then you get all kinds of hum and ground loop problems.

My original mention of the connection from the center lugs of the volume control would normally be if you were to use your Philco as a volume & tone control to the inputs of your Magnavox amplifier. Except with this particular Philco stereo, it looks like you wouldn't have any tone control when tapping from the center lugs of the volume control. Just volume or gain from whatever you have connected to the Philco's inputs.

There are also other options. If you want separate bass & treble controls instead of the dreaded single tone control, look for a separate tube powered preamplifier. They are all over the place. Scott, Fisher, Pilot, and Dynaco are some that come to mind. Some console pulls and maybe even an original preamp or tuner/preamp that was made with your model Magnavox. Do you still have any of the controls that came with your Magnavox? Was it the model which used the passive preamp controls or the tube powered preamp controls?

 

[Harlon]

Thanks,

I do have the complete console for the Magnavox, I believe they are passive controls in the receiver section, at this point I'm trying to get the most for my learning buck out of my Philco.

My long term goal is to have two working consoles. The Magnavox refurbed and the Philco as a preamp with another found amp and speakers from another console.

As long as you can tolerate the questions.

What makes the difference in tapping in at the #2 and #7 on V7 vs R33/C44 and R41/C50. Is it just this design that won't allow linking in to the 6eu7s on the 9304-20. Seems a shame to miss this added layer of amplification and jumping straight to the output tubes.

If I make this conversion how do I need to handle the 175V and 220V sources running to the output stage. Can they just be cut at C1a and C1b. If I cut the one end after I tap R33/C44 and R41/C50 and the other at the aforementioned electrolytics will it effectively remove the output stage.

Thanks again.

 

[gadget73]

This really isn't a great candidate for making a preamp. If you want to keep the tone control (which is really not so much a tone stack as a treble cut control), the output voltage is going to be significantly high. The tone pot is the last thing in the amplifier chain ahead of the output tube. The output voltage there is probably close to 10x what the Magnavox amp wants to make full rated output. The volume control will barely have any useful travel with that setup.

If you're OK with losing the tone control, all you need is output from the center wiper of the loudness pot. That should give a reasonable voltage level to drive the Magnavox to full output without grossly over-doing it.

Honestly, I'd see about picking up something closer to the original Magnavox tuner/preamp for something like this. The signal levels would be closer to right, and you wouldn't be using a pair of output tubes as a current sink to draw down the power supply voltages sufficiently for it to work right.



Just a mention about "losing" the extra amp stage. Notice the lack of negative feedback. Don't need the extra gain on this.

 

[Harlon]

Looks like I'll shelve the preamp idea.

With what I've got in the Philco would there be any way to get more out of the amp. After recapping, it has a nice raw sound that works well with the old blues and mid-80s hardcore/punk I am partial too, but it is too limited on the bass end to do justice to anything with any real depth. If I play anything which needs any bass and turn up the volume it just goes kinda flat.

During the recap I upped coupling caps C43 and C49 from .047 to .1. My son and I sampled .047, .068 and .1 and agreed that we got the best sound from the .1.

I'm pretty much up for any ideas. There is a decent amount of room on the chassis. Could adding in another layer of amplification before or after V7 improve things. I have a spare 6eu7 and socket that I could use if it would be feasible. Only limitation is $$$. I can get caps and the like past the wife but at this juncture new transformers or the like are no goes.

In the end the philco was a freebie and whatever I can learn during this process will be beneficial.

 

[gadget73]

more amplification isn't really going to do it. Its very limited in output power, and the transformers. This is also something of an odd duck in how it makes its third channel for bass reproduction. Not many amps pulled off 3 channels with 2 output tubes. its very much an economy measure honestly.

You are running this in stock 3 channel mode right? It won't do much for low end without the center.

 

[Harlon]

You are running this in stock 3 channel mode right? It won't do much for low end without the center.

Ish. I have the speakers wired per the schematic, but for the Philco I'm extremely limited as to speakers. The console only had the two original 4" speakers and one 8" radio shack speaker.

Please refrain from exclaiming in disdain that that's why I don't have any bass response.

My other project is a Magnavox with 12 inch woofers and the bell horn tweeters. I ran the Philco through these this afternoon and was pleasantly surprised.

Mostly, I'm looking to improve the Philco where possible and learn as much as I can. I've torn down and reassembled it twice now and have kind of fallen in love with it. It sounds like it's not a good candidate but I'd love to continue to tweak it and hot rod it up. There's just not a lot to it and it has become easier and easier to work on. Eventually I'll find a donor console for better speakers and be ready to go.

Thanks.

 

[s-petersen]

To my inquiry, would it be possible to cut down the Philco further and utilize the preamp stage as a preamp for the 9304-20 and Micromatic TT. I thought it would be as simple as disconnecting the preamp outputs running from the balance pot to pin two on the output tubes. Unfortunately that was a failure. Along with a few other options I tried.

I've attached a photo of the amp section of the schematic in SAMs and the one inside the console.

Any assistance would be greatly appreciated.

How did the attempt fail? Did you try to tap between the ,047 cap,and 470k resistor?and use the - on the filter cap for output ground
and pull the output tubes?

I just realized,which schematic is the philco? is it is the first one?

 

[Harlon]

How did the attempt fail? Did you try to tap between the ,047 cap,and 470k resistor?and use the - on the filter cap for output ground
and pull the output tubes?

I just realized,which schematic is the philco? is it is the first one?

Both are the philco. The one is a photo of the sams and the watermarked one came out of the console cabinet.

I tried to tap after the balance pot. The .047 cap ties to the 470k resistor which ties to the balance pot which ties to #2 on the output tubes. I disconnected the lines running from the balance to the output tubes at the tubes. I then attached female RCA receptacles directly to these lines and did a direct link to the 9304-20 input Jack's. I didn't remove the output tubes on the philco or run a line to ground. As I recall all I got was a really loud hum. I also tried running the leads directly off the plate of the preamp tube.

Through this thread I was trying to find out if this adventure was possible and the consensus was that the philco was a bad candidate if I wanted to keep the tone and balance controls. If it is possible and all I needed to do was ground the lines I'll give it another try.

Mostly I'm just looking for a good life for what's left of this integrated amp. It's been a lot of okay I tried that, now what can I do. I've had a lot of fun for not a lot of money and I'd rather not see this old thing end up buried in the garage.

 

[s-petersen]

For the "heck of it" try what I suggested. The controls may not be as effective,but they still may work.You could always make a stand alone amp out of the Philco.