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 Post subject: Help with Zenith amp Phase Inverter & rectifier tube type
PostPosted: Mar Tue 26, 2019 10:40 pm 
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Hello folks,

From my previous amp topic I realized that it is indeed a tube power supply for tube amp and converting it to a power amp will need some component and audio transformer etc. So, along with that I had also a Zenith twin 6V6 tube power amplifier from a Zenith 12H093 cabinet radio. I don't have the entire unit, only the amplifier section. I've currently removed two rotten wires(the insulation started to come part only when it was touched) and changed the film caps.The electrolytic are a bit leaky but working. The seller of the amp had told me that it was removed from a working unit. So, I had a feeling it would not blow up. Also the Phase Inverter tube was cracked and had the chalky color. Unfortunately in the unit which I had received, Zenith seems to have used a different unknown miniature tube. In the schematics they used a 6J5GT. I've attached a picture of the broken tube. I've a feeling it could be a 6C4 but not sure. The broken tube had a faint rubbed of Zenith label but there is no clue about the type of tube. I saw another ad on eBay for the same amp, but it has the original 6J5GT tube itself. Another tube that was missing was the 5Y3GT rectifier.

I've a few questions:
1. First one is regarding the rectifier tube. Can I use a 5U4GB tube instead of the 5Y3GT? I'm aware the peak of the later can go unto 1400V and the 5U4GB only up to 400V. But as per schematics the output is only 350V, so when I tried the 5U4GB rectifier did glow and produce an output but I felt the voltage was lower than whats expected.

2. Second question is regarding the type of phase inverter (PI)tube that I can use. Can I use a 6C4 here? If I want to go with the original 6J5GT design then I'll need to remove and replace the tube socket and then use it.

3. Also what type of audio output transformer should I get? I mean the impedance I should loo for. Could anyone suggest a link to one as an example?

I'm aware that someone in the past had asked some questions regarding the same amp. But because data on this unit was scare people found it harder to give a definite answer on certain things. Hope the info here is useful.

Thanks in advance! :D


Attachments:
Zenith 12H093- twin 6V6  Power amp.jpg
Zenith 12H093- twin 6V6 Power amp.jpg [ 358.71 KiB | Viewed 1219 times ]
Zenith 1.jpg
Zenith 1.jpg [ 807.22 KiB | Viewed 1219 times ]
Zenith 2.jpg
Zenith 2.jpg [ 392.14 KiB | Viewed 1219 times ]
Schematics-Full.jpg
Schematics-Full.jpg [ 378.11 KiB | Viewed 1219 times ]
Schematics- Amp only.jpg
Schematics- Amp only.jpg [ 325.22 KiB | Viewed 1219 times ]
Broken tube- Phase Inverter PI.jpg
Broken tube- Phase Inverter PI.jpg [ 374.31 KiB | Viewed 1219 times ]

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Last edited by Vintage Collector on Mar Wed 27, 2019 6:16 am, edited 3 times in total.
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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 12:02 am 
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Most likely a 6C4 , compare pin connections .


https://frank.pocnet.net/sheets/049/6/6C4.pdf

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 12:57 am 
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I would not use a 5U4 in there because your power transformer will run hotter due to the higher filament demand. The 5Y3 can handle the 6V6 pair and a 6C4 no sweat.


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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 2:10 am 
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Later production did use a 6C4 instead of the 6J5.

Agree that you should not use anything other than a 5Y3 for the rectifier.

Some versions did not use the field coil type speaker, but had a wirewound resistor under the chassis in place of the field coil and those had a PM speaker.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 2:44 am 
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You have your numbers wrong on the 5u4, PIV is 1550v. As said it is not good for this amp due to increased filament current needed. Replace that old filter caps before it shorts out and takes the transformer with it.


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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 6:01 am 
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Also here is a unit similar to mine but with the original 6J5GT Phase inverter like in the schematics. But in this unit the big Zenith 450V 40MF cap is missing, along with the two high wattage ceramic resistors connected to the filter cap like in mine. But in my unit it is used instead of the choke like Mr. Detrola has mentioned above. But the unit in the link seems to have none of that. Strange! :o
https://www.ebay.com/itm/1940s-Zenith-T ... 2300657543

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 6:05 am 
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battradio@ wrote:
Most likely a 6C4 , compare pin connections .


https://frank.pocnet.net/sheets/049/6/6C4.pdf

I'll take a look at the voltages and confirm. From visual inspections the connections seem to match a 6C4.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 6:09 am 
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Audioman wrote:
I would not use a 5U4 in there because your power transformer will run hotter due to the higher filament demand. The 5Y3 can handle the 6V6 pair and a 6C4 no sweat.


Yes, I did notice that. The 5U4GT does draw an extra 1A for the filaments. But I had run it only for a few seconds since I didn't want to risk any damage. The entire power transformer is rated for 110W at 117VAC (says on the transformer shell).

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 6:12 am 
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Mr. Detrola wrote:
Later production did use a 6C4 instead of the 6J5.

Agree that you should not use anything other than a 5Y3 for the rectifier.

Some versions did not use the field coil type speaker, but had a wirewound resistor under the chassis in place of the field coil and those had a PM speaker.


Any particular reason they switched to the 6C4? Like longer life??

Thanks that you mentioned about the speaker. I had missed in the original post one question I wanted to ask. I'll add it too. As per the schematics the output transformer attached to this particular speaker has a 600 ohm center tapped transformer. But you have also mentioned mine could have been the PM type speaker. Could you suggest a link to a transformer that could be suitable for this amp?

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 6:14 am 
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Lou deGonzague wrote:
Replace that old filter caps before it shorts out and takes the transformer with it.


That's no.1 on the to do list before I fire this up the next time! :D The only reason I fired up was because the seller said it was running inside a unit when pulled.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Wed 27, 2019 9:55 pm 
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Also one thing that bugged me from the beginning is that the input RCA jack is connected to the green-green wire on the primary side of the power transformer. Wouldn't this cause the hum and other noise to be introduced this way rather than a separate input audio transformer ?

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Thu 28, 2019 12:30 pm 
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Mr. Detrola wrote:
Some versions did not use the field coil type speaker, but had a wirewound resistor under the chassis in place of the field coil and those had a PM speaker.


I found a link here on this site posted by @Philco Fan. I think this is the PM speaker mod you were talking about. Right?

Attachment:
Zenith Amp mod- Schematic.jpg
Zenith Amp mod- Schematic.jpg [ 136.66 KiB | Viewed 1124 times ]


Attachment:
Zenith data mod.jpg
Zenith data mod.jpg [ 165.87 KiB | Viewed 1124 times ]


Also for the audio speaker I should look for a 600 ohm to 8 ohm transformer?

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Thu 28, 2019 9:34 pm 
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First you must determine what the impedance of the speaker you will be using is going to be.

P-P 6V6's want a 10K plate-to-plate load so you need a transformer which will reflect a 10K CT primary impedance at whatever speaker impedance you place on the secondary.

The 600Ω resistance figure shown on schematics is the DC resistance of the original transformer and has no relationship at all to it's impedance as seen by the output tubes nor does it have any relationship to the primary DC resistance reading of any transformer that might be suitable as a replacement.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Fri 29, 2019 4:51 pm 
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First thing I would add is a fuse!
Perhaps I missed one in the schematic, I only glanced at it for a moment.

I once subbed a 5U4 for a 5AR4 in a nice EL84 PP stereo power amp. I knew it required an extra amp for the filament, but it was intended only as a test. I had planned to use a 5R4 and add a fuse.
It sounded so nice I listened to it for a couple of hours.
Then it had no sound at all. Shorted power transformer. I didn't think it would happen so quickly!

Best of luck on your project.

Rick


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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Fri 29, 2019 7:34 pm 
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6C4 is a better tube because .. it only draws 0.15 amps, half as much current as the 6J5.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Fri 29, 2019 7:42 pm 
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Vintage Collector wrote:
Also one thing that bugged me from the beginning is that the input RCA jack is connected to the green-green wire on the primary side of the power transformer. Wouldn't this cause the hum and other noise to be introduced this way rather than a separate input audio transformer ?
Right, the engineers would never come up with a circuit that increased hum. Looking at the schematic, the input is on its own winding, not on the power input winding. That is a hum-bucking circuit by the looks of it. Another reason to take care of the transformer, as you won't easily find that winding in a replacement.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Sat 30, 2019 5:55 pm 
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Mr. Detrola wrote:
First you must determine what the impedance of the speaker you will be using is going to be.

P-P 6V6's want a 10K plate-to-plate load so you need a transformer which will reflect a 10K CT primary impedance at whatever speaker impedance you place on the secondary.

The 600Ω resistance figure shown on schematics is the DC resistance of the original transformer and has no relationship at all to it's impedance as seen by the output tubes nor does it have any relationship to the primary DC resistance reading of any transformer that might be suitable as a replacement.


Hi Dennis,

Do you think the transformer shown in the below link will be a good choice. I've on of these with me. It a 25W new sensor corp. NSC-41318-T transformer.

https://www.worthpoint.com/worthopedia/ ... -149157870

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Sat 30, 2019 6:04 pm 
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DrRick wrote:
First thing I would add is a fuse!
Perhaps I missed one in the schematic, I only glanced at it for a moment.

I once subbed a 5U4 for a 5AR4 in a nice EL84 PP stereo power amp. I knew it required an extra amp for the filament, but it was intended only as a test. I had planned to use a 5R4 and add a fuse.
It sounded so nice I listened to it for a couple of hours.
Then it had no sound at all. Shorted power transformer. I didn't think it would happen so quickly!

Best of luck on your project.

Rick


Hi Rick,

Yes, I wanted to add a fuse. A little bit of googling shows the transformer MODEL (95-921 100 watts) is 340-0-340 @ 95ma, 5V @ 2A , and 6.3V @2.5A.

Yeah, these transformers aren't so forgiving at times especially if its more than 500mA if run for a long time. For few hours! :shock:

I would never run it for that long unless I was dead sure the transformer can handle it. I have already ordered an NOS 5Y3GT. I do have a couple of power transformers. But here in my case a short transformer is extremely bad news! As you have seen in the below comments mentioned by westcoastjohn. The input winding is within the power transformer itself. Also the amp is so tightly packed that unless I mount a audio output transformer on the sides there is no way. That would look pretty ugly!

Thanks, I've also ordered the 6C4. So, the only thing now I'm waiting to confirm is the type of audio output transformer that I should use.

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Last edited by Vintage Collector on Mar Sat 30, 2019 9:50 pm, edited 1 time in total.

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Sat 30, 2019 6:05 pm 
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westcoastjohn wrote:
6C4 is a better tube because .. it only draws 0.15 amps, half as much current as the 6J5.


That sounds good! A little less stress on that old guy(power transformer) :D

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 Post subject: Re: Help with Zenith amp Phase Inverter & rectifier tube typ
PostPosted: Mar Sat 30, 2019 6:08 pm 
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westcoastjohn wrote:
Right, the engineers would never come up with a circuit that increased hum. Looking at the schematic, the input is on its own winding, not on the power input winding. That is a hum-bucking circuit by the looks of it. Another reason to take care of the transformer, as you won't easily find that winding in a replacement.


Yes, from the circuit I saw that it is a separate winding. Sorry, if my. question wasn't precise. But wouldn't the 60Hz from the line intrude a hum to the input coil. Since its coupled on to the power transformer?

I'll not put this to full use until I get all the required tubes and audio output transformers and verify its ready for everyday use.

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