First Timer needs help

[Eli Duttman]

Originally posted by mgalvez
Rman, is this amp worth modding? I just want a an amp with a clean sound. My listening area is small, so I don't think I need more than a few watts. I am going to start testing today.

Dude,

A look at the schematic shows that amp to be LOFI, sorry. Pentode mode and no loop NFB is (IMO) BAD. The O/P trafos are quite small, which is indicative of poor bass performance. Passing the AC power cord near an O/P trafo is a NO-NO.

All is not lost. The 6AQ5 is an EXCELLENT tube (essentially a 6V6 in a 7 pin mini package), which makes a very linear triode. The power trafo appears to be reasonably substantial. Visit a nearby Walmart and buy a large, plain Aluminum, cake pan. The pan will be your new chassis. Between a hand held drill and a nibbling tool, you will be able to make the drop thru opening the power trafo mounts in.

I suggest you follow the example of the DECWARE SE84, for signal topology. Like the SV83 used in the SE84, the 6AQ5 is a "12" W. tube. Expect approx. 2 W. of O/P in triode mode. The 12AX7 has too much gain, once the tone controls are scrapped. Use a 12AY7 for voltage amplification duty.

You can keep the OEM O/P trafos, if you limit bass extension. An affordable upgrade is the EDCOR XSE10-8-8K.

The B+ supply in a SE amp must be WELL filtered. The simple cap. I/P filter shown is inadequate. It's easy enough to improve on what's there. Use a 5V4GA as the rectifier and save the 5Y3 for another project. You MUST take the "raw" B+ from pin 8 of the rectifier socket, when using a 5V4. Use a 15 muF. electrolytic cap. as the 1st filter element. Follow the 1st cap. with a 6 H. Triad C-14X choke, which is available from Allied Electronics. Follow the choke with a 100 muF. 'lytic. A RC decoupling section follows the CLC filter to power the 12AY7 plates.

[Ty_Bower]

Originally posted by Eli Duttman
A look at the schematic shows that amp to be LOFI, sorry...

Good advice on how to salvage the glass and iron, and rebuild a decent amplifier. However, I suspect most beginners would be discouraged by such a project.

Success is far more likely if he can first get the original amp working again, even if the performance is marginal. After he has a taste, there will be enough motivation to "do it right" on the second pass.

[Eli Duttman]

Success is far more likely if he can first get the original amp working again, even if the performance is marginal.

Ty,

Following that path, I suggest the following.

Replace or deactivate ALL OEM passive parts. A pair of individual L/R 100 KOhm log. taper pots. replaces the volume, balance, and tone control circuitry. Alpha brand Carbon parts from Mouser Electronics are inexpensive and quite suitable. The OEM tone controls are guano and the parts values were chosen to mate with the high impedance of a ceramic phono cartridge, not a CDP.

The issue of excessive 12AX7 gain has to be dealt with. Use a 12AY7 instead of the 'X7. 330 Ohm Carbon composition grid stopper resistors are (IMO) a good idea. Mouser carries inexpensive Xicon brand Carbon comp. resistors.

Knock out the phono I/P connector. Put a rubber grommet into the opening that's exposed. A pair of "captive" cables soldered directly to the volume control pots. will connect to the CDP.

Since this amp will never win a beauty contest, let's do something about the proximity of the power cord to an O/P trafo. Put the new, separate, L/R volume controls in the openings now occupied by the treble and loudness controls. Bring a modern 3 wire power cord in thru the opening occupied by the bass control. Put a toggle switch for turning power on/off in the opening occupied by the balance control. BTW, the labeled OEM faceplate might be removeable.

[Eli Duttman]

Can I make this amp sound good with speakers such as these?

Dude,

Those Dayton MTM speakers appear to be a good value, BUT the little RCA amp has neither the power O/P nor the damping factor to be a good match. I suggest you look into an open baffle design that employs a single driver whose efficiency is in the mid 90s.

[mgalvez]

Hey, I really appreciate everyone's help. I am going to pick up some books today to learn a little more about electronics. I think I might just pick up a kit and start fresh. Maybe a k-12 or something like this
http://www.s5electronics.com/l16stereo.html
Looks that might be a good place to start, then something I can mod. I don't think I know enough to rebuild the amp with different tubes and such without a schematic.

[Eli Duttman]

I don't think I know enough to rebuild the amp with different tubes and such without a schematic.

Dude,

You DO have a schematic. It's the RCA original.

In my previous post I suggested simplifiication. The I/Ps are connected to the "tops" of the new volume controls. The wipers of the controls are connected to the voltage amplifier grid stopper resistors. All the QUESTIONABLE circuitry from I/P to voltage gain triode is discarded.

The 12AX7 and 12AY7 are closely related. They have identical pin outs and similar electrical characteristics. The big difference between the 2 types is amplification factor (mu). The mu of the 'X7 is 100, while the mu of the 'Y7 is 40. Changes in the associated passive parts values are not essential. Just remove the 'X7 and insert the 'Y7.

[Iain McNeill]

Eli's right on track.

If you're going to replace all the caps and resistors anyway (possibly faster than diagnosing the bad ones) why mot simplify the rebuild and pick up all the benefits.

The fun's in getting it working, with whatever you can buy, beg, borrow. You can always improve things again tomorrow.

[mgalvez]

Eli, thanks. That makes much more sense. I am just getting started so I need someone to hold my hand. I just picked up a beginning electronics book so I am going to be on my way. This is the funnest part of a new hobby, picking up tidbits and letting the fog clear. I picked up a chasis fuse (no idea what value fuse to use.) For power cord I was thinking of using one from an old computer power supply with its plug. I need to get some RCA femals and some speaker posts. Once again, thanks for helping a noob!

[Eli Duttman]

Originally posted by mgalvez
Eli, thanks. That makes much more sense. I am just getting started so I need someone to hold my hand. I just picked up a beginning electronics book so I am going to be on my way. This is the funnest part of a new hobby, picking up tidbits and letting the fog clear. I picked up a chasis fuse (no idea what value fuse to use.) For power cord I was thinking of using one from an old computer power supply with its plug. I need to get some RCA femals and some speaker posts. Once again, thanks for helping a noob!

Dude,

Think about the space available for mounting on that tiny chassis. It's very limited. I don't see where either RCA females or binding posts can go.

I suggested "captive" I/P cables previously, because of the limited space. Buy a pair of Canare F10 RCA males and several feet of Mogami W2549 "twinax" shielded cable from Markertek. You will use the Mogami shielded cable in both interconnect and chassis wire service.

An old computer power cord most definitely can be pressed into service. Cut the IEC female connector off. Crimp and solder a ring terminal onto the end of the safety ground wire.

BTW, don't forget rubber grommets where "captive" cabling passes thru chassis holes. Also, be sure to tie strain relief knots on the "captive" wiring, inside the chassis.

Before you order ANY parts, let's make sure the list for each vendor is complete. Shipping charges can eat you alive.

[SHiFTY]

I built a similar tiny amp with a 6BM8/ECL82, it sounded pretty good, and with a bit of inverse feedback it actually was a nice amp to listen to, definitely more than the sum of its parts.

I concur with the other poster, the best way to improve this unit would be to totally strip all the old worn out electronic componentry and start again with a nice simple circuit, with a single volume control at the beginning. You will use the exact same tubes, transformers, but a nice simple circuit will sound better and be very easy to build. You might be able to re-use the can capacitor, but if it gets warm then it will need replacement. All other components should be new, the old ones will be shot. Tubes will be fine, it's always the passive components which don't last!

A random search turned up this circuit that should work reasonably well, and take you all of a couple of days max to build, excluding a nice wooden or aluminium chassis. From: http://www.angelfire.com/vt/audio/