DCAudioDIY.com

DC Area Audio DIYer's Community
It is currently March 29th, 2024, 8:30 am

All times are UTC - 5 hours [ DST ]




Post new topic Reply to topic  [ 8 posts ] 
Author Message
PostPosted: April 17th, 2014, 5:36 pm 
Offline
User avatar

Joined: December 14th, 2013, 2:19 pm
Posts: 948
With the intention of rebuilding an SCA35 into an integrated amp for my TV, I needed a remote volume control. After much consternation (yes, price is a big factor!) I bought a Chinese GA-5 off of an eBay seller. $44 & $12 shipping from Hong Kong.

http://www.ebay.com/itm/121288717326?ss ... 1497.l2648

I haven't yet passed music through it, however, a couple of observations to guide anyone interested (and, no, I'd not expect this to be in the same league as P&G pots, Goldpoint/naked Vishay stepped attenuators, LDR attenuators, Slagle autoformers, etc.) in a solution where remote control volume is desired at a cheap price.

The important item, the pot is an Alps 4-gang 100k motorized pot. It is four gang because the unit supports one balanced input. Since I have no intention of using the balanced function, it is a simple task to parallel two wipers per channel, giving a 50k pot and minimize those rectification issues associated with wipers on tracks. Gold plated (flashed?) contacts for signal wire connections and a separate control board, IR module and pot. Takes up a lot of space but not unworkable.

The remote control is included.

The mute function simply rotates the pot to the zero position, albeit slowly.

The unit supports 5 inputs, at least it is supposed to. In the one I have, input 4 is engaged whenever input 5 (balanced) is engaged. There is no miswiring or evidence of shorted PC traces, so I assume the problem is in the IC controlling the relays. In any event, mine is just a 4 input setup, which suits me just fine, so the flaw is NBD for me. Who knows, other units may not have this problem.

Also included is a rotary enocder, used to cycle between the inputs. It mounts in a 1/4" (6mm) hole, so you can also have front panel chassis control of the inputs. LED drivers are built-in, and a cable included, so your indicator LEDs can be very simply connected to the unit. Standoffs are included for the main board.

9-12VAC is required. Bridge rectifier and LM7805 are on board. I suppose, you may be able to sneak by with a 6.3VAC supply IF you swap out the 7805 for a low-dropout unit and the bridge for Schottkys, but it will be close.

I'd submit more pics, but had to reduce size and resolution of this one to post it. If you want more pics or info, please let me know.

Stuart


Attachments:
GA-5.jpg
GA-5.jpg [ 1.22 MiB | Viewed 20392 times ]
Top
 Profile  
 
PostPosted: April 18th, 2014, 11:23 am 
Offline

Joined: March 12th, 2013, 1:49 pm
Posts: 214
Hi, Stuart, what procedure did you use to rebuild your SCA35? I recently picked up one and I am restoring using Dave Gillespie's board, as well as some input from Michael Samra and Eli Duttman. How do you like the sound of the SCA35?


Top
 Profile  
 
PostPosted: April 18th, 2014, 4:39 pm 
Offline
User avatar

Joined: December 14th, 2013, 2:19 pm
Posts: 948
Randy,

Project is underway, not yet complete.

Easy questions first: love aspects of the sound of the SCA35. Big soundstage, airy and revealing. Hard to find a bad sounding EL84 amp. Was amazed when I first listened to it that it sounded so good. I could hear everything the recording engineer was doing. The unit had lots of problems: hum due to failing P/S caps and one broken hum reduction pot. One output tube different from the others and two mismatched 7199s. The 12AX7s were not Telefunkens. Still, except for really lousy bass (a problem even in properly working Dyna stuff because of high DCR small core power transformers) because largely of power supply issues, the thing really had some amazing attributes.

Before listening to it, I was going to fix it and flip it. Glad I held on to it. However, I am a butcher.

I intended to "restore" it; get it working as designed, but with P/S cap upgrades. Well, since building the efficient little speakers for the living room, I realized this fits the bill almost perfectly: all in one box, suitable for living room display, the right amount of power, more than decent sound, flexibility for general purpose use. But it has its shortcomings: no remote control for volume or signal selection; no 3.5mm front input jack for computer/iPod/phone/etc. music input; useless phono section with low and high level magnetic inputs and ceramic inputs; a poor bias method causing output tubes to not run in the desired range and excessive distortion; the need for 7199 input tubes; horrid RCA connectors spaced so closely that only the worst cables can connect to it; and an equally bad terminal strip for connecting speakers with a completely useless center channel output feature.

The challenge was to make this as simple as possible for the operator (not always me), useful and with the best sound I can squeeze into the chassis. The end result is a total gut, reusing only the transformers (yes, even the crappy power transformer, since nothing else will fit. :>(

I took ideas from Tubelab, Yaniger and Pimm.

The output stage is UL PP using 6HB6 tubes ($1 at ESRC). The input/driver is similar to Yangier's Red Light District, except the 1st stage has a Pimm cascoded CCS for a plate load, this allowing me to use the available B+ (Yaniger used a higher voltage B+ for the input stage). Between the RLD Concertina inverter and the 6BH6s are a pair of Mosfets in source follower mode, DC coupled to G1. To make the PowerDrive circuit work, I've added a +/-37VDC supply, used for the driver power and bias.


Two chokes, 1.5H and 14H are used in the power supply, along with bypassed 330uF 400V caps (3). The rectifier is a pair of Cree Schottkys. Sorry, NO room or power for a VT rectifier. Output terminals are cheap Chinese binding posts on standard 3/4" spacing (4R-0-8R). Input jacks are cheap Philmore from Handmade, actually pretty nice for cheapo! Fewer of them means wider spacing, mounted on phenolic in the original slots.

Bias pots are nice 25k pots, rear panel mounted with pin-type test points in the area of the original "hum" pots. Added IEC power cord connector.

Of course, the GA-5 is my volume control. The front panel will contain Volume pot//Selector switch//Power switch//Tape-Input switch//auxiliary 3.5mm phone jack, IR sensor and LEDs for input and power status.

The rear panel has inputs: phono, tuner/TV, CD, tape in, tape out and preamp out/main amp input (after volume pot).

The phono stage is very simple. 1st stage is a paralleled section 12AT7, 2mA, 120V on plate, CCS plate load with LED bias. Trying for maximum gain from this tube. Following is a relatively high impedance all-in-one passive RIAA, ~272k series resistor, etc. Second stage is cap coupled to the first (PPFX) and is another paralleled section 12AT7/CCS/LED. Output is a big PP cap, ~4uF). I may add a Mosfet follower if the 12AT7 seems to be buckling under the 45k output load.

It's all pretty simple. Guy's Quad CCS boards are big time/money/space savers. Bolt the TO220 devices to the chassis for the little heat sinking that they need. Board hangs from the TO220 devices.

I'm a couple of weeks away from music (I hope). If you like, I can post progress pictures as it unfolds.
Unfortunately, it won't look, or be anything like a stock SCA35. My bad.

As to Samra, Duttman and Gillespie. Don't know Gillespie (but his EFB bias was inspirational!!). But listening to Eli Duttman, you won't go wrong. Good luck!!

Stuart


Top
 Profile  
 
PostPosted: April 18th, 2014, 8:13 pm 
Offline

Joined: March 12th, 2013, 1:49 pm
Posts: 214
thanks, Stuart, my own SCA35 will be several months out, time is tight and needed on some other projects to complete, but it would be interesting to compare the two after we get them done to see how the differing topological changes sound!


Top
 Profile  
 
PostPosted: April 18th, 2014, 9:04 pm 
Offline

Joined: March 12th, 2013, 11:12 am
Posts: 738
I am also a big PP EL84 fan. I have a lightly modded ST35 with the 565 OTs as well as some Dave M PPEL 84 monoblocks. These little amps punch way above their weigh class!


Top
 Profile  
 
PostPosted: April 18th, 2014, 10:39 pm 
Offline
Site Admin
User avatar

Joined: February 28th, 2013, 10:38 am
Posts: 1682
Take a look at the Acrosound 20/20 schematic. The bias could use some help as the tubes are run too hot, but it's a great circuit....

Roscoe

_________________
I can explain it to you, but I can’t understand it for you.


Top
 Profile  
 
PostPosted: April 18th, 2014, 11:14 pm 
Offline

Joined: March 12th, 2013, 1:49 pm
Posts: 214
Roscoe Primrose wrote:
Take a look at the Acrosound 20/20 schematic. The bias could use some help as the tubes are run too hot, but it's a great circuit....

Roscoe


Yes, Dave Gillespie addresses the bias issue in his EFB, which I plan on using in my rebuild, as well as doing away with the 7199s for 6U8s. Stuart, I am unfamiliar with the 6HB6, what are they?


Top
 Profile  
 
PostPosted: April 19th, 2014, 8:41 am 
Offline
User avatar

Joined: December 14th, 2013, 2:19 pm
Posts: 948
Randy,

The 6HA6/6HB6 is a sweep tube with a different noval inout than the EL84. It has twice the transconductance of the EL84, a higher plate voltage rating and at least the same plate dissipation. The 10 watt published is a sweep rating which is always lower than the "audio" service rating.

The great news is that they are on the dollar menu at ESRC. Those are NOS USA tubes. I bought 60 to get started. About what you'd pay for four EL84s. We will know hoe they sound in a couple of weeks. If they don't work out, I can rewire the sockets and plug in EL84s. I'd only be out $60. Win-win.


Top
 Profile  
 
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 8 posts ] 

All times are UTC - 5 hours [ DST ]


Who is online

Users browsing this forum: No registered users and 38 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB® Forum Software © phpBB Group