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Philco K1629 Upgrade Options
Philco K1629 Upgrade Options
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Old 8th May 2015, 11:48 PM   #1
Harlon is offline Harlon  United States
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Default Philco K1629 Upgrade Options

I have a Philco integrated amp that I've been using to teach myself how to work on. It is a simple single 12ax7 driving tone, volume and balance pots through two 6bq5s. I've recapped, rewired and reresistored it. I recapped it exclusively with orange drops, other than the electrolytics. It sounds alright and much better than pre-recap, but I'd like to see if I can make it sound better.

I'm looking for any suggestions/ideas for improving the circuit and improving the overall performance of the amp, and looking to learn more about how these amps work while implementing your ideas.

My ideas thus far:

1). What would adding a negative feedback loop do to the performance?
2). What would adding another preamp layer do? I have a good spare 6eu7 from another stereo.
3). What would replacing the three small output transformers with one or two beefier units yield?
4). What would putting in a bigger transformer pushing more volts do?
Any ideas/suggestions would be appreciated. I'm looking to get the most out of the circuit and learn more about how it works in the process.

I've attached two copies of the schematic.

Thanks,
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Old 9th May 2015, 01:59 AM   #2
Eli Duttman is offline Eli Duttman  United States
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It looks like you have a pretty decent power trafo. IMO, that's worth saving, but much of the rest is, unfortunately, guano. The cheapskates have mono bass PP. YUCK! I've seen that kind of trashy circuitry before and remain singularly unimpressed.

Better sound, you said.

Buy proper SE O/P "iron", with both adequate power handling and bandwidth. Edcor's GXSE15-6-5K will work with both 8 and 4 Ω speakers and the price is quite reasonable. The power handling capability in excess of the 6BQ5's capability means the "iron" will reach deeper into the bass region than the spec. sheet suggests.

Simplify the signal path. Get rid of shaping circuitry and retain only listening level control. Your, thinking about a GNFB loop is spot on. Both linearity and damping factor are positively impacted. A 12AX7 section voltage amplifier should prove adequate.

The Edcor O/P trafo linked above offers you a choice between UL and full pentode mode O/P. If you go full pentode mode, regulate 6BQ5 g2 B+ at about 250 V.

Another improvement you can consider is CLC filtering the B+. The 6CA4 rectifier is quite competent. Take advantage of its capabilities and use a 47 μF. 1st filter capacitor.
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Old 9th May 2015, 09:48 AM   #3
Harlon is offline Harlon  United States
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Quote:
Originally Posted by Eli Duttman View Post
Better sound, you said.

Buy proper SE O/P "iron", with both adequate power handling and bandwidth. Edcor's GXSE15-6-5K will work with both 8 and 4 Ω speakers and the price is quite reasonable. The power handling capability in excess of the 6BQ5's capability means the "iron" will reach deeper into the bass region than the spec. sheet suggests.
I should add that I'm listening to the amp through a set of speakers and crossover from a 63 Magnavox tube console. 12 inch woofers and horn tweeters.
Don't know the efficiency db rating on these but I can only assume that using a crossover they aren't up to the needs of a SE amp. With that said to keep stereo capability would I have to use at least two opts to replace the three existing? Would a pair of the GXPP15-6-5s fill the need here?

Quote:
Originally Posted by Eli Duttman View Post
Another improvement you can consider is CLC filtering the B+. The 6CA4 rectifier is quite competent. Take advantage of its capabilities and use a 47 μF. 1st filter capacitor.
By CLC, are you recommending incorporating a filter choke after the first filter capacitor, replacing the 1K 5W resistor. I'm on board, but not sure how to calculate the ohm value for the choke. My other amp uses a 55ohm choke to yield a 5V drop from 330V to 325V. Here I need to drop 45V from 220V to 175V. Doing the math would I need a roughly 500ohm choke?
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Old 9th May 2015, 02:15 PM   #4
monty78pig is offline monty78pig  England
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Philco K1629 Upgrade Options
You could probably lose the tone controls. Get better OPTs (EDCOR are good), use LM317 current sources to bias the cathodes use a CLC filter in the power supply to reduce hum.

The first thing I would do would be to give each 6BQ5 it's own cathode load (a 317 CCS bypassed with a 1000uF capacitor is ideal).

I would use this choke if I were you https://edcorusa.com/cxc100-5h-200ma. Connect it with a 47uF capacitor in front of it just upstream of your 220V rail.
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Old 9th May 2015, 05:42 PM   #5
Harlon is offline Harlon  United States
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Default Breaking it down, starting with the power supply and filtering section

Thanks for the input.
I need to break this down a little bit, starting at the power supply.

At the PT I have probably 120 coming in at the wall being boosted by secondary to ~190. This runs to the 6ca4 and gets boosted again to ~220. These are given. With the suggested mods I'm not impacting any of these figures. Also, I can assume my modern line voltage pushes all these numbers up around 5 volts.

I believe the suggestion is to run this 220-225 volts first through a 47mf electrolytic into a 5h 200mA choke. At 200 mA and 58ohms this will drop my voltage ~11.5 volts. This is going to drop my b+ somewhere in the 210-215 volt range.

Couple questions at this point.

1). If I run 2 Edcor GXPP15 4-5k Output Trans, will that 210-215 on each center tap be enough to get full operation from the transformers and 6bq5s?

2). If I want to stay true to the old school technology in the rest of the power supply(translation: run from more modern technology I don't understand) would I pick up the rest of the rcrc from the existing system? Right now after the 220 line out it runs 1000ohm/5w resistor to 30mf electrolytic to 1800ohm/5w resistor to 10mf electrolytic. This yields resistor voltage drops of 45 and 50 volts respectively.

Thanks for the help.
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Old 9th May 2015, 07:32 PM   #6
Eli Duttman is offline Eli Duttman  United States
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Yes, you do need a pair of O/P trafos (1/channel).

You should get about 250 V. of B+ out of the 6CA4/EZ81. Perhaps a tad low, but nothing terrible. To squeeze a few volts more out of the PSU, switch to SS B+ rectification using 2X Cree C3D02060F high PIV Schottky diodes.

CLC filtration is quite sufficient for O/P tube B+. All that's additionally needed is RC section decoupling, to feed the small signal circuitry.

The screen grid in a 6BQ5/EL84 is plenty tough. So, UL mode "finals" make considerable sense, given the "short" B+ rail. Unfortunately, the 12AX7 triode is a wimp, which may not be up to driving a UL mode O/P tube. That issue can be dealt with by "borrowing" the ZVN0545A buffer from the phono preamp schematic I've uploaded. If you FET buffer the 'X7 voltage amplifier, employing "fixed" bias O/P tubes is definitely feasible. Yes, you have to build a low voltage negative rail to use "fixed" bias, but you gain the opportunity to squeeze 5 honest W. out of each O/P tube.
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Old 9th May 2015, 08:34 PM   #7
Harlon is offline Harlon  United States
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Quote:
Originally Posted by Eli Duttman View Post
The screen grid in a 6BQ5/EL84 is plenty tough. So, UL mode "finals" make considerable sense, given the "short" B+ rail. Unfortunately, the 12AX7 triode is a wimp, which may not be up to driving a UL mode O/P tube. That issue can be dealt with by "borrowing" the ZVN0545A buffer from the phono preamp schematic I've uploaded. If you FET buffer the 'X7 voltage amplifier, employing "fixed" bias O/P tubes is definitely feasible. Yes, you have to build a low voltage negative rail to use "fixed" bias, but you gain the opportunity to squeeze 5 honest W. out of each O/P tube.
I have on hand a 12at7. Could I use the existing 12ax7 as a gain stage and then utilize the 12at7 as a driver/phase splitter to feed the output tubes?

If at all possible I'd like to maintain the "integrated" original nature of the amp. I want to improve it and beef it up, but I'd like to keep it free of newer bits and pieces of tech. Also, if at all possible I want to maintain the preamp tone and volume aspects.

It may be silly, but I'm aiming for an integrated that can still drive it's turntable and an iPad and I can tweak treble or bass to suit the music I'm listening to at the time.
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Old 9th May 2015, 11:54 PM   #8
Eli Duttman is offline Eli Duttman  United States
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Those cheapy consoles used piezoelectric cartridges, along with low grade record changers. Can you say record eaters? An old Technics, Pioneer, BIC, etc. TT off EBone or CL, a modest MM cartridge, and a phono preamp are what you need to enjoy LPs, without filing for bankruptcy. While opamp based, Jim Hagerman's Bugle2 is certainly sufficient for a "budget" system. Please observe that the Bugle2 employs passive RIAA EQ, just like the tweaked RCA setup I previously uploaded.

You certainly can retain an "integrated" amp. Adhere to the KISS principle. Use a pair (1/channel) of Mouser stock # 31VJ501-F3 pots. to control listening level and channel to channel balance. Except (possibly) for source selection switching, use no other controls.
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Old 10th May 2015, 12:07 AM   #9
monty78pig is offline monty78pig  England
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Philco K1629 Upgrade Options
Lads, you need to remember passive eq is only a good idea when you have obscene amounts of headroom, such as you have with valve preamps. Not so good for op-amps when you only have +-12 or so output voltage.

I agree with Eli, those autochangers you find in most older valve equipment are complete drek. With 78s I still shouldn't want to use them ! They track at about disgusting levels (7+ grams) and are about as compliant as a brick :P .
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Old 10th May 2015, 01:58 AM   #10
michaelsamra is offline michaelsamra  United States
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Default There is something you will notice about these amps and that is

They are biamped with an extra transformer that feeds the bass from both channels into one trafo.This is why it is best to put two new opt trafos on like what was suggested. Depending on how far you want to go and being you are using this amp as an educational tool,you can utilize the bass trafo if you want to get clever just for experimenting and playing with crossover points.
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