SSS #2 Started

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groovtubin
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Re: SSS #2 Started

Post by groovtubin »

ic-racer wrote:
dreric wrote:Well ........................ passed the smoke test!

But........................ no sound :(

Weird.....................High pitched between a squeal and a hum barely audible but changes pitch as you move the master / level pot. Also changes pitch as you manipulate the bias pot.

I know that the squealing, loudly, can be reversed secondaries on the OT, could this be happening?? Thought as to where to start looking first?

Eric
This is where I find it nice to have the in/out jacks so you can isolate the problem to either the preamp or the power amp.
OPEN the PI entrance, SLAVE in a jack/test signal!!!

jim@Omegaamps
ecisthebest
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Re: SSS #2 Started

Post by ecisthebest »

dreric wrote:Well ........................ passed the smoke test!

But........................ no sound :(

Weird.....................High pitched between a squeal and a hum barely audible but changes pitch as you move the master / level pot. Also changes pitch as you manipulate the bias pot.

I know that the squealing, loudly, can be reversed secondaries on the OT, could this be happening?? Thought as to where to start looking first?

Eric
i had this on an amp. the feedback loop is what was causing my problem. don't know how to fix aside of killing the feedback loop, but try poking components and wires in the feedback loop with a stick?
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crbowman
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Re: SSS #2 Started

Post by crbowman »

Do you have access to a scope?
If so, you can plug a test signal into the input and start probing. Maybe start with the first grid and then to each subsequent grid until you lose your signal.
Then go backwards to the low voltage side of each coupling cap until you find where the signal stops. Should help you narrow it down a bit.
If you don't have a scope you can use a simple signal probe. There are threads around here someplace with instructions as to how to build one, but I'm not having any luck searching for it right now.
<i> "I've suffered for my music. Now it's your turn."</i>
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dreric
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Re: SSS #2 Started

Post by dreric »

Where I left off, the amp passed smoke but had a faint high pitched hum and no out output. Bias was unstable and would not go below 90ma. (BTW............ that's enough voltage to get the tubes hot enough to melt the insulation right off reverb cables, just saying)

Bummer..............I was thinking I needed a nice Manzamp.

So, I've been working to get this beast tamed and I've made enough changes to post an update:

1. I took the high pitched hum to mean oscillation and I reviewed my OT wiring. Sure enough I had the secondaries going to the speaker output reversed (black to tip). Correcting the wiring (green to tip) took care of the hum and I started getting some output. Bias dropped slightly to 70ma.

2. I changed the grid resistors on the power tubes from the value listed, 1K5 to the more conventional 5K1. No change in the amp. A few people have PMed me suggesting that the 1K5 is an error on the schematic.

3. I messed around with the bias resistor and tail resistor until I got a range that would give me a stable 32ma. Bias resistor was changed from 3K3 to 2K2 and the tail went from 22k to a whopping 91K.

4. Changing the resistor values also got the bias voltage at V7 to -345. the listed value is -328.

**I have to consistently remember the the schematic voltages were taken with the amp at 102VAC. I don't know if I should expect my voltages, with the amp at 120VAC should be 18volts higher, I guess we'll find out**

These changes got the power amp working!

I'm out of time but I'll post voltages for the whole amp tomorrow.

Eric
Eric
1949 Zenith, Zenith Toggle Recoil, Zenith 55 & 440
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dreric
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Re: SSS #2 Started

Post by dreric »

Here's the voltages. Comparison to (values) listed on the hand darwn schematic, which were taken with the amp running on 102VAC:

Node----My Value-(Schem)---V----pin 1vdc----(Schem)----pin 6vdc (schem)

A--------447 --------(298)----V1----232--------(189)---------237-------- (185)

B--------344--------(301)-----V2----352--------(187)---------352-------- (187)

B -------344--------(301)-----V3----342--------(184)---------342--------- (302)

C------- 342--------(300)-----V4----229--------(211)---------231---------(207)

G------- 444--------(???)-----V5----400--------(331)---------400

D-------419---------(392)----V6----411--------(245)---------412----------(241)

E-------444---------(378)----V7----442--------(378)---------442

H------447---------(378) goes to pin 3, plates of V8-V11, amp has 442 on the plates.

Feedback is not only welcomed but encouraged!

Happy Super Bowl!

Eric
Last edited by dreric on Sun Feb 03, 2013 8:31 pm, edited 1 time in total.
Eric
1949 Zenith, Zenith Toggle Recoil, Zenith 55 & 440
grtamp
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Re: SSS #2 Started

Post by grtamp »

Hi Dreric , sorry , where is the 22k tail res. in the schematics? Is a resistor you have add?

Voltage node A it seems to me 298V on the schematic! not 378V.
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dreric
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Re: SSS #2 Started

Post by dreric »

grtamp wrote:Hi Dreric , sorry , where is the 22k tail res. in the schematics? Is a resistor you have add?

Voltage node A it seems to me 298V on the schematic! not 378V.
Hey,

Thanks for catching the node A value, I updated the chart.

The bias circut on the schematic doesn't seem to show a bias pot (10K) or the 22K tail resistor but the layout does. I wired it as per the layout so I could bias the power tubes.

Eric
Eric
1949 Zenith, Zenith Toggle Recoil, Zenith 55 & 440
grtamp
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Re: SSS #2 Started

Post by grtamp »

Hi thanks , so you have replaced 22k with 91K!! that's big difference!!

have you tryed to increase the capacitor bias supply? 10uF seems low value to me!
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Structo
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Re: SSS #2 Started

Post by Structo »

Yeah I would think at least 100uF for the bias cap.

Some think more than one bias cap is a good thing too, as well as safety resistors on the bias pot in case the wiper fails.
Tom

Don't let that smoke out!
ATL Strat
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Re: SSS #2 Started

Post by ATL Strat »

Any progress on this build? It looks great!
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dreric
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Re: SSS #2 Started

Post by dreric »

ATL Strat wrote:Any progress on this build? It looks great!
Thanks for your interest.........

I've got next steps figured out:

1. Get the dropping string worked out so I have better voltages.

2. Figure out how to use my oscilloscope so I can hammer out where I'm loosing the signal, pretty sure it's V1B. The layout and schematics get funky there.

Going to have some time this weekend!

Eric
Eric
1949 Zenith, Zenith Toggle Recoil, Zenith 55 & 440
ATL Strat
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Re: SSS #2 Started

Post by ATL Strat »

dreric wrote:
ATL Strat wrote:Any progress on this build? It looks great!
Thanks for your interest.........

I've got next steps figured out:

1. Get the dropping string worked out so I have better voltages.

2. Figure out how to use my oscilloscope so I can hammer out where I'm loosing the signal, pretty sure it's V1B. The layout and schematics get funky there.

Going to have some time this weekend!

Eric
Awesome. I'd start by tracking down the signal - you can fine tune the voltages later (unless they are way off).

Check out this vid for some hands on guidance on tracking down a signal in a guitar amp. http://youtu.be/6dwsAhe63cw

Good luck!
ATL Strat
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Re: SSS #2 Started

Post by ATL Strat »

I noticed that the 470k resistors from power section grid resistors to ground were ommitted on Aaron's layout (they show up in the schematics). Can someone tell me what the purpose of these resistors is? Is it something unique to a Cathode Follower setup?
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Reeltarded
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Re: SSS #2 Started

Post by Reeltarded »

from grids to ground would be a divider. Haven't seen schem or layout..
Signatures have a 255 character limit that I could abuse, but I am not Cecil B. DeMille.
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dreric
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Re: SSS #2 Started

Post by dreric »

ATL Strat wrote:I noticed that the 470k resistors from power section grid resistors to ground were ommitted on Aaron's layout (they show up in the schematics). Can someone tell me what the purpose of these resistors is? Is it something unique to a Cathode Follower setup?
Thanks for pointing this out and I'm embarrassed that I missed this detail. Especially since I have put some thought into this section.

Originally, I used the 1.5K resistors which are on the schematic I didn't catch the 470Ks. Then, at the advise of a few, I switched the grid resistors to the more conventional 5.1K, but it always bothered me.

It's bothered me because either the tech who drew the schematic was either consistently dyslexic, substituting 1K5 for 5K1, four times on the schematic or I was missing something. So I think I'll try going back to the 1K5 and adding the 470K and see what happens........... mojo?

Just to check in on progress:

1. Kind of figured out how to use my oscilloscope and signal generator and I've traced the signal through the amp.

2. In the process, I found one eyelet which I didn't solder and two erroneous connections.

3. With those fixed I get signal through the amp but the signal is very weak, whisper level.

So I got signal but no power.

I'm thinking that the next step is to swap the 5K1 for the 1K5 and add the 470K to ground maybe that has to be for the cathode follower to work....... and I'm thinking that I should get my voltages in line with the data from the schematic.

Thanks for looking.

Eric
Eric
1949 Zenith, Zenith Toggle Recoil, Zenith 55 & 440
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