Newcomb conversion

[SpareRibs]

Hello,
I just obtained a Newcomb E10-B Pathfinder PA. I was wondering how hard it would be to convert itto a guitar amplifier? I know it will need a preamp my only question is about the wiring situation with themike and phono input as I understand the routing is divided between two tubes, also the phono input hasa system designed for a phono cartridge. Thank You in advance!

[olddawg]

Here is what I would do. Figure out the tube complement and what the B+ is. From that you can decide what type of guitar amp you can convert it to. Then gut it and install a new turret/eyelet board w/new components, new ceramic tube sockets, new jacks, and new pots. If it is using oddball preamp tubes switch to 12ax7s. If you need to bring up the B+ a little you can use an SS rectifier. If you need more heater current you can use an LED or neon power indicator in most cases off of the AC line rather than a 6v filament lamp that is usually the case. If you take this approach rather than trying to modify the existing circuit you will save yourself a lot of time, headaches, and will have an actual usable guitar amp. It really isn't that expensive. You have the expensive parts.

[SpareRibs]

Conversion

Here is what I would do. Figure out the tube complement and what the B+ is. From that you can decide what type of guitar amp you can convert it to. Then gut it and install a new turret/eyelet board w/new components, new ceramic tube sockets, new jacks, and new pots. If it is using oddball preamp tubes switch to 12ax7s. If you need to bring up the B+ a little you can use an SS rectifier. If you need more heater current you can use an LED or neon power indicator in most cases off of the AC line rather than a 6v filament lamp that is usually the case. If you take this approach rather than trying to modify the existing circuit you will save yourself a lot of time, headaches, and will have an actual usable guitar amp. It really isn't that expensive. You have the expensive parts.

Hello,
That being the case I may as well start with a flat piece of sheet metal !!!!!!!!!!!!

[madkatb]

I googled the schematic for that amp and it sure has a strange phase inverter, 6au6 and half of a 12ax7. Try changing the components around the first half of the 12ax7 to something cathode biased and use it as your preamp tube. Good place to start fiddling anyway.
I also googled "6au6 12ax7 6v6 pp" and came up with this if you wanted to try modding it.
6au6 12ax7 6v6 pp image by printer2_photo on Photobucket

[olddawg]

Hello,
That being the case I may as well start with a flat piece of sheet metal !!!!!!!!!!!!

Not really. The transformers are the important and expensive part. It wasn't designed as a guitar amp. All of the electrolytic caps are probably bad. You will spend more time and money trying to make the old stuff usable and reliable in most cases, then have lackluster results. It's better to start fresh with a known good design. It isn't difficult to make a tag board with a few components. Ceramic sockets etc, are cheap.

[SpareRibs]

Conversion

I googled the schematic for that amp and it sure has a strange phase inverter, 6au6 and half of a 12ax7. Try changing the components around the first half of the 12ax7 to something cathode biased and use it as your preamp tube. Good place to start fiddling anyway.
I also googled "6au6 12ax7 6v6 pp" and came up with this if you wanted to try modding it.
6au6 12ax7 6v6 pp image by printer2_photo on Photobucket

Hello,
Thank you very much for your reply. As you were concerned enough to pull up the schematic.Without disregarding your suggestion I was thinking if going through the microphone jack,doing away with the phono jack and phono volume would that not get me through to the entire PI or in other words. Wouldn't that send the signal on the same path. Past that point I could change some resistor values add some cathode bypass capacitors to increase the signal.
I am not really experienced in electronics. I read a Gerald Weber book while converting an old Magnavox amp into a guitar amplifier, It worked very well. One of the problems was that the book tellsyou how to do improvements, it just does not explain why they work. I have to say it is still a very good book
Thanks again

[madkatb]

What olddawg said about the transformers and everything else is very true. However, if you're not too familiar with electronics this amp may be a good way to get some practical experience. Read everything you can about guitar amps and heed every safety warning. There's dangerous voltages inside.
There are other postings about this same amp on this forum, use the search function. A good place to start reading may also be Merlin Blencowe's Valvewizard pages How to design valve guitar amplifiers
You could disconnect the wire/resistor from the wiper of the phono volume and play through the mic input. That's as close as you're going to get without making some mods.

[gldtp99]

Hello,
I just obtained a Newcomb E10-B Pathfinder PA. I was wondering how hard it would be to convert itto a guitar amplifier? I know it will need a preamp my only question is about the wiring situation with themike and phono input as I understand the routing is divided between two tubes, also the phono input hasa system designed for a phono cartridge. Thank You in advance!

I have one of these Newcombs that i converted to a Fender 5E3-based guitar head----- nice iron in these---- 4/8/16 ohm secondary taps on the OT.
I kept the orig rec tube--- new JJ 40/20/20/20uF at 500V cap can--- 12ax7's in preamp.
i just gutted the orig preamp and built what i wanted into the available space--- sorry i don't have pics of this one, i built it several yrs ago.
But here's some pics of a single ended 10 watter i built using Bogen CHB10A transformers---amp has a modded Marshall #36 mod-type preamp---built on a blank chassis from Watts---- Marshall 20 watt type headcab---faceplate to soon be ordered---

Pics: Mule 10 Watt Single Ended pictures by gldtp99 - Photobucket

Sound Clip: zw3 by gldtp99 on SoundCloud - Create, record and share your sounds for free

I shared the info about the SE 10 watter to show that using old PA amps as a base to build guitar amps is limited only by one's imagination----sometimes i use the orig PA amp chassis, sometimes i pull the iron, etc and use a new blank chassis to build whatever idea i currently have--- most of them work out, but sometimes my ideas don't pan out as well as i'd hoped--- so i just do it over-------- Experimenting is Fun !!!!..........................gldtp99

[SpareRibs]

Modification

I googled the schematic for that amp and it sure has a strange phase inverter, 6au6 and half of a 12ax7. Try changing the components around the first half of the 12ax7 to something cathode biased and use it as your preamp tube. Good place to start fiddling anyway.
I also googled "6au6 12ax7 6v6 pp" and came up with this if you wanted to try modding it.
6au6 12ax7 6v6 pp image by printer2_photo on Photobucket

Hello,
I pulled up the Photobucket schematic (Thank You). It is going to be great, it has the same tube configuration but the resistors,capacitors, and tone stack are more in line with guitar amplifier values. I am laying it out on paper now. This will be the best place to start then if necessary I can change values to tune it in. Thanks again

[SpareRibs]

Tone Stack

Hello,
OK I have done a lot of research on this PA. After powering it up and putting the guitar signal through it, I am getting the RIAA sound (Thin spindly sound,overlaid with tons of bass). Having converted a record player to a guitar amp before and fighting all the problems with changing resistors, and capacitors I decidedthis time to take the short cut by just replacing the tone stack as the first modification. That will get rid of the RIAA circuit in one operation.
The schematic is on (ampix.com). I intend to replace the tone circuit with the Fender Deluxe (5E3)I am routing the input jack to the grid of the 12AX7 and ground to the cathode. The tone circuit goes between the plate of the first half of the 12AX7 and the grid of the 6AU6.
Does this sound like a reasonable approach? I have all the capacitors to do the cap job, and I intend to replace the power cord with a grounded one.
Thank You in advance for any further input!!!!!!

[gldtp99]

Sounds like a good idea---- but remember that a real Fender 5e3 circuit uses a 12ay7 in V1---- a 12ax7 here will sound bloated and over-gained---so you might adjust stock values on the 5e3 V1 setup (larger cathode resistor, maybe smaller cathode bypass cap).
Since you're going this far, consider getting rid of the 6au6 and wiring up the Tweed Deluxe 12ax7 Gain Stage/Cathodyne PI---this is one of the classic, great sounding guitar circuits.
I keep mine in my workshop and have pulled it out several times over the yrs to show people what a 10 watt amp can sound like--- it pushes a 4x12 cab very well---i sometimes get people asking about amp builds who insist that they need at least 100 watts---some really do, but most don't---they just don't know it.
If you need help with values i can open mine up and see what i did...................gldtp99

[SpareRibs]

Hello,
I increased the value on some of the resistors (Cathode 6AU6). I also added a few in the signal path. I really added values to a lot of the capacitors. I rewired the treble so it dumps some of it to ground. I am going to experiment with the negative feedback loop, possibly using a potentiometer so as to be able to add and subtract it.
I can see the possibility of running 4-12s that you mentioned, it really cranks out. I have a little AC ripple but I am going to add some capacitance in the main filter circuit.
All in all it is coming out very well. Thank you very much for your interest and advice!!!!!!!!!!!!!!!!!!!!!!!!!!!!

[SpareRibs]

Hello,
Does every tube need a bypass capacitor on the cathode resistor ?

[sgelectric]

Spareribs,

Every tube does not *need* a bypass capacitor on its cathode resistor.
In guitar amps bypass capacitors are typically used to increase gain (reduce degenerative feedback) and to tailor frequency response of a given stage.

SG